Is there a way to "pass on" UI events.

You can simply add those events to foreground objects too. So they will work as if foreground objects aren't there.

Edit: My comment seemed a bit confusing to me so i felt a need to make clarification: you don't need to write a new event for each event trigger. You can use same event for multiple triggers.

I found the Scroll Rect wasn't receiving it's events if a container object had an Event Trigger component. It didn't matter if the MouseDown, OnDrag (or whichever events the Scroll Rect is using) were being consumed - the Event Trigger sucks all the events dry and doesn't seem to let anything through.

I was using an asset that made heavy use of the Event Trigger, and didn't want to rewrite that. So I solved this by decompiling the Event Trigger and making my own version that included only the events I needed. Then it was simple to change the asset's references from "EventTrigger" to "MyEventTrigger"

Code follows. Notice most of the event interfaces are commented out.

 using System;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.Events;
 using UnityEngine.Serialization;
 
 namespace UnityEngine.EventSystems
 {
     /// <summary>
     /// 
     /// <para>
     /// Receives events from the EventSystem and calls registered functions for each event.
     /// </para>
     /// 
     /// </summary>
     [AddComponentMenu("Event/Event Trigger")]
     public class MyEventTrigger : MonoBehaviour, 
         //IEventSystemHandler, 
         IPointerEnterHandler, 
         IPointerExitHandler, 
         //IPointerDownHandler, 
         //IPointerUpHandler, 
         IPointerClickHandler
         //IBeginDragHandler, 
         //IInitializePotentialDragHandler, 
         //IDragHandler, 
         //IEndDragHandler, 
         //IDropHandler, 
         //IScrollHandler, 
         //IUpdateSelectedHandler, 
         //ISelectHandler, 
         //IDeselectHandler, 
         //IMoveHandler, 
         //ISubmitHandler, 
         //ICancelHandler
     {
         [SerializeField]
         [FormerlySerializedAs("delegates")]
         private List<MyEventTrigger.Entry> m_Delegates;
         /// <summary>
         /// 
         /// <para>
         /// All the functions registered in this EventTrigger.
         /// </para>
         /// 
         /// </summary>
         [Obsolete("Please use triggers instead (UnityUpgradable) -> triggers", true)]
         public List<MyEventTrigger.Entry> delegates;
 
         /// <summary>
         /// 
         /// <para>
         /// All the functions registered in this EventTrigger.
         /// </para>
         /// 
         /// </summary>
         public List<MyEventTrigger.Entry> triggers
         {
             get
             {
                 if (this.m_Delegates == null)
                     this.m_Delegates = new List<MyEventTrigger.Entry>();
                 return this.m_Delegates;
             }
             set
             {
                 this.m_Delegates = value;
             }
         }
 
         protected MyEventTrigger()
         {
         }
 
         private void Execute(EventTriggerType id, BaseEventData eventData)
         {
             int index = 0;
             for (int count = this.triggers.Count; index < count; ++index)
             {
                 MyEventTrigger.Entry entry = this.triggers[index];
                 if (entry.eventID == id && entry.callback != null)
                     entry.callback.Invoke(eventData);
             }
         }
 
         /// <summary>
         /// 
         /// <para>
         /// See [[IPointerEnterHandler.OnPointerEnter]].
         /// </para>
         /// 
         /// </summary>
         /// <param name="eventData"/>
         public virtual void OnPointerEnter(PointerEventData eventData)
         {
             this.Execute(EventTriggerType.PointerEnter, (BaseEventData)eventData);
         }
 
         /// <summary>
         /// 
         /// <para>
         /// See [[IPointerExitHandler.OnPointerExit]].
         /// </para>
         /// 
         /// </summary>
         /// <param name="eventData"/>
         public virtual void OnPointerExit(PointerEventData eventData)
         {
             this.Execute(EventTriggerType.PointerExit, (BaseEventData)eventData);
         }
 
         /// <summary>
         /// 
         /// <para>
         /// See [[IDragHandler.OnDrag]].
         /// </para>
         /// 
         /// </summary>
         /// <param name="eventData"/>
         public virtual void OnDrag(PointerEventData eventData)
         {
             this.Execute(EventTriggerType.Drag, (BaseEventData)eventData);
         }
 
         /// <summary>
         /// 
         /// <para>
         /// See [[IDropHandler.OnDrop]].
         /// </para>
         /// 
         /// </summary>
         /// <param name="eventData"/>
         public virtual void OnDrop(PointerEventData eventData)
         {
             this.Execute(EventTriggerType.Drop, (BaseEventData)eventData);
         }
 
         /// <summary>
         /// 
         /// <para>
         /// See [[IPointerDownHandler.OnPointerDown]].
         /// </para>
         /// 
         /// </summary>
         /// <param name="eventData"/>
         public virtual void OnPointerDown(PointerEventData eventData)
         {
             this.Execute(EventTriggerType.PointerDown, (BaseEventData)eventData);
         }
 
         /// <summary>
         /// 
         /// <para>
         /// See [[IPointerUpHandler.OnPointerUp]].
         /// </para>
         /// 
         /// </summary>
         /// <param name="eventData"/>
         public virtual void OnPointerUp(PointerEventData eventData)
         {
             this.Execute(EventTriggerType.PointerUp, (BaseEventData)eventData);
         }
 
         /// <summary>
         /// 
         /// <para>
         /// See [[IPointerClickHandler.OnPointerClick]].
         /// </para>
         /// 
         /// </summary>
         /// <param name="eventData"/>
         public virtual void OnPointerClick(PointerEventData eventData)
         {
             this.Execute(EventTriggerType.PointerClick, (BaseEventData)eventData);
         }
 
         /// <summary>
         /// 
         /// <para>
         /// See [[ISelectHandler.OnSelect]].
         /// </para>
         /// 
         /// </summary>
         /// <param name="eventData"/>
         public virtual void OnSelect(BaseEventData eventData)
         {
             this.Execute(EventTriggerType.Select, eventData);
         }
 
         /// <summary>
         /// 
         /// <para>
         /// See [[IDeselectHandler.OnDeselect]].
         /// </para>
         /// 
         /// </summary>
         /// <param name="eventData"/>
         public virtual void OnDeselect(BaseEventData eventData)
         {
             this.Execute(EventTriggerType.Deselect, eventData);
         }
 
         /// <summary>
         /// 
         /// <para>
         /// See [[IScrollHandler.OnScroll]].
         /// </para>
         /// 
         /// </summary>
         /// <param name="eventData"/>
         public virtual void OnScroll(PointerEventData eventData)
         {
             this.Execute(EventTriggerType.Scroll, (BaseEventData)eventData);
         }
 
         /// <summary>
         /// 
         /// <para>
         /// See [[IMoveHandler.OnMove]].
         /// </para>
         /// 
         /// </summary>
         /// <param name="eventData"/>
         public virtual void OnMove(AxisEventData eventData)
         {
             this.Execute(EventTriggerType.Move, (BaseEventData)eventData);
         }
 
         /// <summary>
         /// 
         /// <para>
         /// See [[IUpdateSelectedHandler.OnUpdateSelected]].
         /// </para>
         /// 
         /// </summary>
         /// <param name="eventData"/>
         public virtual void OnUpdateSelected(BaseEventData eventData)
         {
             this.Execute(EventTriggerType.UpdateSelected, eventData);
         }
 
         /// <summary>
         /// 
         /// <para>
         /// Called by a [[BaseInputModule]] when a drag has been found but before it is valid to begin the drag.
         /// </para>
         /// 
         /// </summary>
         /// <param name="eventData"/>
         public virtual void OnInitializePotentialDrag(PointerEventData eventData)
         {
             this.Execute(EventTriggerType.InitializePotentialDrag, (BaseEventData)eventData);
         }
 
         /// <summary>
         /// 
         /// <para>
         /// See [[IBeginDragHandler.OnBeginDrag]].
         /// </para>
         /// 
         /// </summary>
         /// <param name="eventData"/>
         public virtual void OnBeginDrag(PointerEventData eventData)
         {
             this.Execute(EventTriggerType.BeginDrag, (BaseEventData)eventData);
         }
 
         /// <summary>
         /// 
         /// <para>
         /// See See [[IBeginDragHandler.OnEndDrag]].
         /// </para>
         /// 
         /// </summary>
         /// <param name="eventData"/>
         public virtual void OnEndDrag(PointerEventData eventData)
         {
             this.Execute(EventTriggerType.EndDrag, (BaseEventData)eventData);
         }
 
         /// <summary>
         /// 
         /// <para>
         /// See [[ISubmitHandler.OnSubmit]].
         /// </para>
         /// 
         /// </summary>
         /// <param name="eventData"/>
         public virtual void OnSubmit(BaseEventData eventData)
         {
             this.Execute(EventTriggerType.Submit, eventData);
         }
 
         /// <summary>
         /// 
         /// <para>
         /// See [[ICancelHandler.OnCancel]].
         /// </para>
         /// 
         /// </summary>
         /// <param name="eventData"/>
         public virtual void OnCancel(BaseEventData eventData)
         {
             this.Execute(EventTriggerType.Cancel, eventData);
         }
 
         /// <summary>
         /// 
         /// <para>
         /// UnityEvent class for Triggers.
         /// </para>
         /// 
         /// </summary>
         [Serializable]
         public class TriggerEvent : UnityEvent<BaseEventData>
         {
         }
 
         /// <summary>
         /// 
         /// <para>
         /// An Entry in the EventSystem delegates list.
         /// </para>
         /// 
         /// </summary>
         [Serializable]
         public class Entry
         {
             /// <summary>
             /// 
             /// <para>
             /// What type of event is the associated callback listening for.
             /// </para>
             /// 
             /// </summary>
             public EventTriggerType eventID = EventTriggerType.PointerClick;
             /// <summary>
             /// 
             /// <para>
             /// The desired [[UnityEvent]] to be Invoked.
             /// </para>
             /// 
             /// </summary>
             public MyEventTrigger.TriggerEvent callback = new MyEventTrigger.TriggerEvent();
         }
     }
 }