For booting OS 9 on a FW800 G4, see my discussion in this thread: http://68kmla.org/forums/viewtopic.php?f=3&t=9967&start=0
Regarding ROM 10.2.1, I have seen it suggested that it contains MDD bug fixes for booting into OS9.
The special 9.8.1 rom that came on a MacTest Pro disc that supposedly allows OSX only macs to boot into OS9 is somewhat of a myth. It only supports OS9 booting on one single OSX only model, the iMac (Flat Panel) 15 inch (and that model did in fact previously support OS9 booting). Those who have tried the 9.8.1 rom on other OSX only G4s have discovered it does not allow them to boot into OS9. However, there was also a special blessed system file called "Custer" on that MacTest Pro CD and I haven't seen anyone do much with this file to see exactly what it does.
There was also a French company that developed a firmware patch called Countdown G5 that allowed OS9 booting on G5s. See http://db.tidbits.com/article/7617.
Regarding ROM 10.2.1, I have seen it suggested that it contains MDD bug fixes for booting into OS9.
The special 9.8.1 rom that came on a MacTest Pro disc that supposedly allows OSX only macs to boot into OS9 is somewhat of a myth. It only supports OS9 booting on one single OSX only model, the iMac (Flat Panel) 15 inch (and that model did in fact previously support OS9 booting). Those who have tried the 9.8.1 rom on other OSX only G4s have discovered it does not allow them to boot into OS9. However, there was also a special blessed system file called "Custer" on that MacTest Pro CD and I haven't seen anyone do much with this file to see exactly what it does.
There was also a French company that developed a firmware patch called Countdown G5 that allowed OS9 booting on G5s. See http://db.tidbits.com/article/7617.