I was able to resurrect a ZD1100 that was stuck in Rescue Mode. The internal USB stick had died. I removed the stick, and repaired it with the "SMI Mass Production Tool" but decided not to use it, since if it died once, I wasn't going to rely on it. I had another, generic 2GB stick lying around, and used that instead. It's nothing special - just some old compact, folding thumb drive I've had in a desk drawer for years. I tried a larger stick, but that produced some complaint about the size, so there must be a narrow range of sizes that will work.
The real key to my success, however, was to ignore the feedback from the web interface, and rely on the console output instead. In particular, the FDISK function of the web interface reports success far too readily. The console output is much more accurate.
I inserted my generic 2GB USB stick, pressed the FDISK button in the Rescue Mode web interface, then watched the progress in the console. When formatting was done, I tried uploading various firmwares via the web interface. At first, I tried newer ones, but the console output showed 'out of memory' errors of various sorts. I went back as far as 9.3.x, and that was small enough to work. Once I had 9.3.x firmware running successfully, I performed the usual sequence of upgrades, restored my config backup, and all is well.
Maybe this can help someone else keep their older, end-of-life hardware running. Obviously, I would not recommend doing this for a box that's under a current service contract or warranty.