[Stuart]

@alfiejmulcahy put up a picture on the blue forum showing exactly how the desk reflection works:

Thank you Studio7, that was the picture I was looking for this morning when I got 'lost' on Google following too many interesting links (as you do). 

The reflection is not a keyed-element, it is a deliberate layer of the VR environment, over and above the physical set. You could paint the top of the desk 'pink with purple polkadots', and the reflection effect would still be maintained by the VR rendering. It's not relying on any green reflection from within the studio.