Hello,
I think I might have found the solution. This at least works on landscape-left and portrait. You have to crop the UVs to feed into the ARKit Passthrough Camera node in the same way that UE4 does it for the background (I copied the logic from AppleARKitVideoOverlay.cpp).
Then for landscape-left:
Or for portrait:
This is not a finished material and just intended as an explanation. It should be optimized by calculating the offset from a BluePrint at BeginPlay and pass it to the material as a parameter. I’ll post the final version once I get it done.