Can I use the Darwin/Mac OS X BootX bootloader to boot NetBSD/macppc?
No, although it should be easier to port to NetBSD/macppc than the linuxppc BootX, since the Darwin BootX uses Open Firmware. See this post for instructions on getting the BootX source code if you’re interested in working on this. This repository also contains nvram patches for machines with Open Firmware 2.4 and earlier, based on the patches in System Disk.