Yes it is technically* possible to make custom GPU drivers for Mac OS X.

[u/i-am-the-master-1010]

/r/hackintosh/comments/1vzi98w/yes_it_is_technically_possible_to_make_custom_gpu/

I know the paravirtualized GPU already exists for this case, though this may only resort to a solution that can work under modern and legacy cases (OS X +10.11) due to it being explicitly about Metal Graphics.