fullscreen webcam texture behind everything?

Follow this guide to create your layers and cameras. then create a guitexture and apply the webcamtexture to that.

http://answers.unity3d.com/questions/9729/how-can-i-display-a-flat-background-2d-image-not-a.html

 public void Initialize(){
     Debug.Log("Initialize");

     BackgroundTexture = gameObject.AddComponent<GUITexture>();
     BackgroundTexture.pixelInset = new Rect(0,0,Screen.width,Screen.height);
             //set up camera
     WebCamDevice[] devices = WebCamTexture.devices;
     string backCamName="";
     for( int i = 0 ; i < devices.Length ; i++ ) {
         Debug.Log("Device:"+devices[i].name+ "IS FRONT FACING:"+devices[i].isFrontFacing);
         
         if (!devices[i].isFrontFacing) {
             backCamName = devices[i].name;
         }
     }
     
     CameraTexture = new WebCamTexture(backCamName,10000,10000,30);
     CameraTexture.Play();
     BackgroundTexture.texture = CameraTexture;
             
 }`

Create another camera that is looking at your textured plane (or just blit it to screen). If you go for the camera approach, just set it to draw before the other cameras you're using by setting its depth. The "normal" camera shouldn't clear color, depth only.