This problem is getting more bizarre.
I booted from the OS 9 universal install CD, and my G4 locked up again!
I ran the Apple hardware test and got a VRAM error code: "disp/15/5". There's nothing wrong with my graphics output, however I do have this
"red tint" issue on 3D applications, however I've seen that on multiple ATi graphics cards, and I've never heard of VRAM going bad over time.
I have a 9000 Pro and a 440 MX on the way.
UPDATE: Pulled out my SSD + adapter, removed my USB hub, issue still happens when booting from CD. Maybe my graphics card really has died? Doesn't quite explain why I can use the Mac just fine with extensions disabled, although maybe it's the ATi drivers trying to communicate with a dead part of the graphics card that is causing the lockups. I'll try disabling the drivers now.
UPDATE 2: Yup, that worked.
Semi-dead video card locks up the entire computer when ATi extensions are enabled. Who woulda thought? Not true...
UPDATE 3: I
found an old post with a guy who was having the same issue. Turns out reseating his video card was enough to get everything working again. Come to think of it, the actual last thing I did to my Mac was to swap my 7500 with a flashed FX 5200; it didn't work but that's beside the point. So I've just reseated my 7500 and everything is working again with all ATi extensions enabled. Whew.