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 macOS is missing?