Initial output from the latest ROM.
It does in fact require OF commands to boot
These may seem very familiar
dev /
" PowerMac 10,1" encode-string " MacRISC3" encode-string encode+ " MacRISC" encode-string encode+ " Power Macintosh" encode-string encode+ " compatibility" property
dev /cpus/PowerPC,G4
80010201 encode-int " cpu-version" property
So here's the thing now. And this is probably going to put a huge wall in the way.
So going through this, I realized something and did some checking.
The one thing that the other G4 systems have that the mini does not is ADB.
Yes there are no physical connectors on anything past the B&W G3 but, they all have a virtual bus via the PMU that is mapped from the usb ports. I have suspected the PMU in the mini (being the only one that is not used in any other G4) would be a problem. The mini has no ADB emulation in it at all so the 68K emulator and other managers will choke because they will not be able to find it. There was a reason the virtual bus was there and it was exactly to support anything older than OS X which does not require this. The G5's will sit in the exact same position as well.
So ultimately the mini is the first system that was really OS X only.
Now this doesn't mean we can't work around it, but we are now talking about firmware level virtual device drivers.
So long story short, I wouldn't hold your breath on the mini (i have been wrong before so don't hold me to this) as it would probably be a while before there is any possibility that it could boot and run.