Heh ok got it booting: it was a bad rc.udev. Quicker to fix than I thought
...but I still want to backup the users and re-install clean. There's other nasties lurking on this system that I don't have the time to debug all of them.
... I guess I'm off to backup my data to a new partition. If I can't get it back to it after I reinstall, I'm sure I'll be back here!