Ignoring layers in UnityGUI

I am trying to ignore an Image component overlay so I can trigger events underneath. This component has to be rendered last.

I would like to catch events on a gameobject that has been rendered before. I thought that I might use the GraphicRaycaster for that by putting the overlay on a different layer and setting the raycaster but I can't get that to work (setting BlockingObjects and BlockingMask doesn't seem to help). I can disable the Image component and then everything works as intended but that obviously is not the solution.

So how do I ignore the newer gameobjects and let the event system work in the gameobjects rendered earlier?

I am using Unity 4.6b20