It looks like the pass through part here was implemented using standard DriverKit interfaces, if I'm not mistaken. That is, the PCIe BAR can already be mapped from the user-space, without any extra modifications to macOS. It's just a matter of VMMs, such as QEMU, adopting this interface in addition to Linux VFIO and the like (unless you're talking about Virtualization.framework, which is kind of a VMM of its own).

What exactly do you feel like macOS is missing?

I’m not very familiar with the specifics of pass through but IIUC only being able to map 1.5gb of active DMA buffers at a time is pretty limiting.