NONE OF THE G4 POWERMACS WITH A 7447A/B CPU CAN BOOT OS9 NATIVELY!
Yes, but this is more a Firmware restriction than a Mac Os 9/ Mac Os ROM problem. The 7447 and 7448 upgrades usually patch the Firmware not Mac Os 9/ Mac Os ROM to properly work.
I heard from another user that Mac OS ROM can indeed affect drivers. I have yet to experience this myself. Albeit he claimed to have used a newer rom and from what I've gathered that 10.2.1 is the latest?
This is definitely a central point. That's the Mac Os ROM 10.2.1 is the way to go because it can startup most if no ALL of the Macs that can run Mac Os 9 and this show us that this rom has the wide range of drivers for a computer startup Mac Os 9...
In fact Mac OS 9.2.2 was released in December 5, 2001. And what did Apple to enable Mac Os 9 in later computers models? : upgrade the Mac Os ROM, until they decided to stop of doing it...
You can startup without Audio or Video drivers (Video cards usually have a very slow "compatible" mode without drivers), but what do you think would happen when the SO try to initialize some motherboard chips with different behavior than expected... by example, a unknown Uninorth (may be the case of Mac Mini) or ATA 133 after a weird bridge (the case of a G4 Server) or a completely unknown Motherboard (the case Of G5)
What happens is a completely system crash.
... and by the way: congratulations for your success.
Don't forget to try the very last nVidia drivers:
http://macos9lives.com/smforum/index.php?topic=1100.0