The port I had in mind is this one [1], and the use case is to make it possible to run 3D accelerated Linux guests on macOS hosts. I may have wrongly believed that it would be useful for macOS guests.
Besides, it is correct that whatever happens on the driver side of things, Apple will have to approve it first (as the other reply pointed out)
macOS guests use Apple’s own ParavirtualizedGraphics framework, which comes with none of the downsides of virtio-gpu. It pipes through serialised Metal at native performance.
I knew virtio support was happening but hadn't seen any info on the GPU bit.