The saga: after the overnight effort, I had a disk image of the hard disk with the problem...and with all my data and applications. I did a system re-install but that didn't fix the problem--the startup process hung before getting to the login screen. I'd had consultations and hand-holding via IM with kd5mdk (thanks again!), but he was away at this point. I called the Apple enterprise support folks again and was told that the next thing to try was to erase the hard disk, then install a clean system. After that I could move stuff (i thought this guy told me both data and applications) from the disk image, and see what I had.
So, I erased the disk and installed a new system, then contemplated my situation. It was mid-afternoon, I had to leave for Denver Saturday, and I really needed the computer with some fairly complicated software pieces, not all of which I had to hand for new installations. I decided I really wanted to try to use Migration Assistant to move substantial parts of my old account over, and I made one more call to Apple and got a different support guy who felt I could only move documents from the damaged disk image--MA would be right out. kd5mdk (consulted for a third opinion) agreed that was the best procedure, but that if I wanted to try using Migration Assistant it *might* work, or I might go a step too far and have to repeat the disk erase and re-install.
I took a deep breath and did it--used MA to move Documents and Applications from my old account, and crossed my fingers as I re-started the computer. And it booted! Rather slowly, and shut downs are slow, so I desperately need to do a new set up properly when I'm home--clean install again, then do the software installs one at a time and discard the accumulated detritus from years of using the Powerbook G4. I still had to work on the VPN installation, and had to re-install AFS from scratch, but the rest of my critical applications are running.
I must mention the yeoman job the iPad did as my chat platform with kd5mdk. Office nannyware blocks all Web chats, the old IM installation I used to have on my Windows box got cleaned off a while back (and I wasn't using it anyway), and if there's a way to make Lotus Sametime talk to other chat systems through our firewalls I don't know about it. The iPad G3 connection avoids the nannyware nicely.