See the Irix prom manpage for more help on PROM commands. Then prepare a second hard disk with NetBSD's FFS and disklabel. The firmware can't currently boot the kernel from an FFS filesystem, though, and you'll need to boot that via the net for now. Make sure you have the crosscompiler first in path. Here's a method to set the ethernet address that works on at least the Indy, and perhaps also other machines with HPC3's.
All disclaimers apply yadda yadda.
If not, you may not want to overwrite whatever is there instead. It's a very good idea to reuse the address the machine was shipped with. Check the back of the machine if you cannot remember it. Power down, wait one full minute and then power up. The PROM tftp client on SGI machines can fail with default NetBSD tftpd settings.