...put your FW800 into Disk Target Mode, connect it to a Mac Os 9 Mac, and reformat its HD with Mac Os 9 drivers as you'll do whith a normal FW external enclosure.
Then you can drag-install a valid system folder on the FW800 HD (still in DTM mode) and restart your Mac os 9 machine whith the HD of the FW800 (still in DTM mode) to test this HD boot.
Shut down both computers, unplug FW cable, cross your fingers and start the FW800.
Good luck.
I think that Mac Os ROM 10.2.1 will do the trick. But I never tried to boot a FW800 with Mac Os 9...
If you need another ROM version, just ask, I have an almost complete colecction
I'll hope to have explained it well...