does this system have a floppy drive? it's a long shot, but assuming the model in question is wired to be able to boot from a floppy at all, you could likely mod an old netbsd install disk to boot, load the scsi driver, and start a tftp server. the server should be small enough that if you lose the actual installer there'd be room for it. i don't remember ever having to hold down any keys to boot from a floppy on any mac capable of doing so.