I just wanted to finish up this story for anybody who found it because they are in the same predicament.
I never found a way to rescue the half-upgraded installation, but I did manage to dump config.xml over my serial console.
Ultimately I downloaded the latest usb stick image with serial console, booted from it, and did a reinstall (over my serial console).
The install was fast, upgraded the whole thing to ZFS, and - much to my surprise - found and used my existing config.xml
I instructed the machine to reboot and again, was extremely surprised to find that my relatively complex configuration was back up and working with no more intervention. I logged into the web interface and found a message saying packages were being automatically installed, and not to change anything in the GUI and that was it.
I'm very impressed at what was a particularly easy and intuitive experience.
Thanks for anyone who offered help!