@ivesen i know you don't want to hear it, but if this were gentoo you could pull a generic libc from tinderbox and unzip it to / and then run the update again. Then again, i don't know that gentoo could fuck up an install this badly. maybe it could.
I assume a reboot doesn't work... any luck on googling the error about the post-hook? maybe there's a patch or something and you can boot a rescue disk (gentoo has 1 ;-) and patch it and run dpkg --whatever the command is to make it re-do.