I know that there was a few failed tries of booting Mac Os 9 on Mac Mini G4 with the iMic "Universal ROM" . I don't have any of this Macs nowadays, but I have a new suggestion if someone one is willing to try. I think the failure is because the G4 CPU is a 7447 or a 7448, so it needs the Openfirware Patch alongside the iMic "Universal ROM".
The Open Firmware commands needed are for just changing in the Device Tree the G4 CPU 7447 by 7455 or any other G4 CPU that the Mac Os 9 System knows about it . As we usually have to do when we use the 7447 or 7448 in a CPU upgrade card.
This commands were published around here... somewhere...

BTW: For the the Mac Os 8.6 fans, with this commands and a easy mod on the Mac Os ROM, may be we can boot the Mac Os 8.6 (the Sawtooth version) in to most of the Macintosh G4 family (up to QuickSilver)

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IMPORTANT: for those who do not want to read the many pages of this topic,
The Mac Mini OS 9.2.2 (v9 ISO Image) for booting the Mac Mini with all updates can be downloaded here:

Mac Mini Install CD Image (Version 9):
http://macos9lives.com/smforum/index.php?topic=4365.msg30502#msg30502
http://macos9lives.com/smforum/index.php?action=downloads;sa=view;down=2
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And, it's changed again.
Now visit http://macos9lives.com/downloads for the download.