I managed to fix the problem by reinstalling the 10.2.5 combo update, rebooting, finding a hosed system, using Target Disk Mode to re-reinstall 10.2.5, becoming unable to even get single-user mode, using Open Firmware to eject the cd in the drive, replacing it with the 10.2 install CD, using Firewire Target Disk mode again to delete enough things to make 2 GB of space, re-installing 10.2, and installing 10.2.5 over that.
...it felt like I was using Windows

Target Disk mode is very cool, although I only ever use it in emergencies.