I gave up on this once I took a look inside the memstick image and compared it to the upgrade image. the memstick contains an ISO, the upgrade contains a tar image of a viable filesystem. I had a known working memstick available, so I slicked the box and reinstalled that way, then added back my config file and was on my way.
of course reverting that way doesn't scale, you need local access. I did figure out where to upload a backup to have it recognized in the web ui, but wasn't sure if trying to roll back a release would succeed from that route.