Use Vuforia VIdeo Feed for Occlusion Shader

I want to use the the video feed in a grab pass shader to create an occlusion material with Vuforia.
I had this working before, over a year ago but it doesn't seem to work now.
I was hoping the standard grabpass shader https://docs.unity3d.com/Manual/SL-GrabPass.html would do it, but I still get the other geometry in it (which I want occluded). I have tried with various Queue values like "Background+100" or "Geometry-100" but I either get black or I get the bg and the other opaque objects.
I noticed the AR Camera can have a render target. Do I need to use that?