The hard way:
Turn off autoboot in Open Firmware, format the disk remembering to include a ~4MB HFS partition at the start for the bootloader and install, reboot using the USB drive's bootstrap to get into your hard drive's installed OS, build and install HFStools to be able to write to that partition so you can copy over the bootloader and a small rescue image, reboot to get into OFW again, turn autoboot back on and tell it to load the bootloader you copied over.
The easy way:
If you have a Mac OS X install media, use the disk utility to create at least three partitions, one being the bootloader partition which can also be a Mac OS X install in its own right if you have the room and want to, then copy the bootloader and small rescue image over to that first partition, leaving the other two for root and swap. Then install and tell Open Firmware to automatically boot via the NetBSD bootloader.
The easy way would not have been that much less convoluted than the hard way.
Turn off autoboot in Open Firmware, format the disk remembering to include a ~4MB HFS partition at the start for the bootloader and install, reboot using the USB drive's bootstrap to get into your hard drive's installed OS, build and install HFStools to be able to write to that partition so you can copy over the bootloader and a small rescue image, reboot to get into OFW again, turn autoboot back on and tell it to load the bootloader you copied over.
The easy way:
If you have a Mac OS X install media, use the disk utility to create at least three partitions, one being the bootloader partition which can also be a Mac OS X install in its own right if you have the room and want to, then copy the bootloader and small rescue image over to that first partition, leaving the other two for root and swap. Then install and tell Open Firmware to automatically boot via the NetBSD bootloader.
The easy way would not have been that much less convoluted than the hard way.