The Mac Pro still has auxiliary power connectors, so you could theoretically install a GPU in it, just not the highest end ones. No drivers so you can't really do anything with them. I do suppose Nvidia could maybe do CUDA drivers for a card, but would Apple even allow it?
@paul IIRC it's entirely a limitation of the apple silicone preventing the OS using additional GPUs. I think virtualization pass through as others mentioned might be the only use.