For those having problems accessing the Mac mini Open Firmware, try using an Apple keyboard.
I found that using an MS and a Logitech keyboard both resulted in no open firmware, yet other commands such as reset PRAM worked fine. Switching to an old Apple G3 keyboard worked fine.
Can get as far as a purple screen with dodgy colours - as others have described. To me this looks more like an incorrect screen resolution or refresh rather than a graphics card problem, but I am no expert.
Mac mini G4 1.42GHz, 1GB RAM
ROM 4.8.9f1
ATI Radeon 9200 32MB, ROM 113-xxxxx-116
Good luck, this is an interesting development!