I did this once to fix a mac mini that experienced a similar failure after applying the 10.4.11 delta update. It requires some familiarity with Terminal.
I booted into single user mode (Hold down Command-S during startup).
Once started up in single-user mode, the file system is available as read only.
Do the following:
/sbin/fsck -fy
/sbin/mount -uw /
Then I used the following instructions to disable the GUI by editing the file /etc/ttys . This way the computer would boot into multi-user mode, just without a graphical interface. This way I had networking available so I could copy the combo updater to the mac mini. Maybe you could avoid all of this by burning the combo updated to a CD - I'm not sure if you can install an OS update in single user mode.
http://oreilly.com/pub/h/348
Once I did that, I ftp'd a copy of the 10.4.11 COMBO update (which since this incident I always use) to the broken computer, from another machine I have.
I mounted the disk image with the following:
hdid MacOSXUpdCombo10.4.11Intel.dmg
which created:
/Volumes/Mac OS X 10.4.11 Combined Update (Intel)
I then ran the following command to install the combo update:
/usr/sbin/installer -pkg /Volumes/Mac\ OS\ X\ 10.4.11\ Combined\ Update\
\(Intel\)/MacOSXUpdCombo10.4.11Intel.pkg -target /Volumes/Macintosh\ HD/
Once it was done installing, the system worked again!
I forget if I had to undo the changes made to /etc/ttys to keep the GUI from starting, or if /etc/ttys was updated by the combo installer and everything was normal after rebooting.