One thing you could try is to delete the Display Preferences file from OS 9, boot into Open Firmware and see if you can set the correct mode:
dev screen
show-modes
set-mode x
Where x=valid mode from show-modes command.
That's a good idea;or at least it would be if not for the second issue the good Admiral is having:
I booted to OS9 with VGA on another monitor and it came up fine.
I shut down and booted to OSX on DVI and it displayed fine.
I booted back to OS9 on DVI and noted that it would display to the point of the "Happy Mac" icon and then go blank.
I rebooted holding shift and got the same results with the "Happy Mac" and then it went blank.
I rebooted with Command-V and the results were also the same with the "Happy Mac" then it went blank. No text appeared.
The Mini (and this may also be ROM-related) is NOT recognizing the Aluminum kbd until the kbd extension is loaded so NO startup key commands work.
This is another one of those fun little known "mysteries" that pop up sometimes in Mac hardware - especially during transitions like PPC > Intel; MacOS > OSX etc.
It's a kind of thing that happens when you don't have the exact correct combination of hardware, software, firmware rev, ROM rev etc.
Figuring out exactly which lil' bit 'o code is the culprit however… ooh, some fun now!
It's probably ROM-related. It: A) Does NOT happen with all Aluminums; B) Happens with both OS9 and OSX.
I have exactly the same issue using an Aluminum kbd with my MDD. But I
love my Aluminum, so…
I took the easy fix route and I just keep an older USB kbd handy to plug in if I really need to get to OF or clean a desktop or whatever.
Bottom line is you gotta get to the Monitor panel and/or prefs to reset them.
Since you can boot OS9 with VGA, I would try booting it up, finding and deleting the Monitor prefs file and rebooting with the DVI connected.
……with ALL digits crossed.