It's been a long time, but I'm pretty sure A/UX didn't use any of the toolbox roms and had it's own drivers (we had the source). A/UX booted from a MacOS partition because the Mac bootloader only understood booting MacOS (and it wasn't writeable with new boot code), so you booted to MacOS, then started SASH, which loaded Unix.

That's exactly the reason. NetBSD uses its own booter for the same purpose, for example.