I have tried with the famous 9.8.1 Mac ROM from the diagnostic CD you mentioned but no luck again... however it comes at handy for future modifications because it already include in the initial part two extra Mac models that are:

PowerMac4,2 (iMac G4/800 MHz - 7450 CPU)
PowerBook4,3 (iBook  G3/900 MHz - 750fx CPU)

There's a thing that I suspect: the Diagnostic CD for G4 MacMini does not more contain a regular (modified) OS9 partition but another type of boot sequence, very similar to the one for the G5 machines and this lead me to look to what kind of processor is inside the OS9 PowerMacs...
Well, putting together the informations from the 68KMLA thread and the MacTracker database I discovered that...

NONE OF THE G4 POWERMACS WITH A 7447A/B CPU CAN BOOT OS9 NATIVELY!

So, what permit to the iBook 800 user in the other thread to boot os9? Simple: althought Apple disabled for this model (that has an ID equal to PowerBook 6,3) the OS9 support they build this machine with a 7455 inside and this is enough for the modified MacROM to load.

So, in the end, to have a universal MacROM file running on every G4 (and maybe G5) the real challenge is disable in the code the CPU check because THIS is the real issue in booting OS9, the text string in the beginning of MacROM it is only the first barricade, once removed all the G4 Macs with a 7455 CPU should be able to run OS9, but the 7447A/B Mac based needs to wait until someone really smart in PPC assembly can patch the ROM to disable this check.

At this point the only lucky users that benefit from the Mac ROM modding could be the iMacs users... this Mac family never adopted a 7447 CPU so the chances to have success in booting OS9 are higher than later PowerBooks, for example.