I just snagged a 1.5GHz G4 Mini on eBay, so I'll officially be able to help with this project once it arrives!
I received my Mini today. Mildly unfortunate - what showed up was a 1.42GHz instead of the 1.5GHz model it was listed as. (I'll bitch at the seller tomorrow.) On the up-side, it has 1GB of RAM.
I went straight into testing a 9.2.2 drive that I've been using for awhile in another machine. It was made from the "Unsupported G4s ASR", and I believe it's the one that I was using in the 9serve. I attached it via firewire so I could leave the OS X install on the internal drive intact. The only thing I swapped out was the Mac OS ROM file. The disk was selected for boot using the option key method.
Amazingly, only a single extension out of everything I normally have enabled broke the boot - "Apple Audio Extension". After disabling that it booted properly, obviously without audio support, but it booted extremely fast. Video detected a single resolution of 1920x1080, which is correct for this monitor, but predictably it's only offering 256-color mode. Something is causing a Finder crash when it reaches the desktop but I have yet to determine what it is. One weird thing is that the Apple menu won't enable the submenu for the Control Panels folder.
There is one strange twist with the disabling of the PMU device. For some reason the system identifies this Mini model as an iBook. Without power management loading the system obviously complains when reaching the desktop that the software is missing. The weird part is that after a few minutes of use a warning came up saying the system was low on power, as if it was running on a battery, and then promptly shut down because it thought it needed to protect itself. I removed the Energy Saver control panel to prevent this from happening again, leading to the missing software warning every boot.
Everything else seems to work. The AppleTalk control panel even states there's a serial port available, presumably because it recognizes the modem.
Now for the coolest part - I'm posting this from Classilla running on this Mini! (This is getting a bit challenging on Classilla because compatibility with this forum is not as good as it used to be.)
Edit - One additional note - The device info for the Radeon 9200 in this thing is as follows:
Device name: "ATY,RockHopper2_A"
Chipset: "ATY,RV280"
Device ID: 5962
ROM: 113-xxxxx-116