Having trouble with a mask shader - any tips welcome!

Hi there. I am trying to use the following technique to mask an object:
https://answers.unity.com/questions/316064/can-i-obscure-an-object-using-an-invisible-object.html
This works beautifully in the editor, but when I enter VR my masking object is black.
Here is the set-up in editor (I am using a cube to obscure the hexagonal shaft here)...

Here it is in the editor with the game running (working fine - shaft hidden)...

And in VR (mask object is black)...

I am guessing this has something to do with depth buffers or single-pass rendering...
Any suggestions would be most welcome!
Thanks,
Dan