I think my KP is suffering from some kind of dissociation syndrome. Despite the battery being close to full when the stuck-on-boot issue occurred, leaving the device plugged in overnight did sort of bring it back to life.
Unfortunately, it doesn't seem to be reading its own Settings. It came back to life showing only the Kindle User Guide (no ebooks) and saying I need to register and connect to WiFi, despite "Clytie's Kindle" and a strong WiFi signal being in the top status bar, and despite its own Settings showing I am logged in and connected.
I bought an ebook and Sent it to that device, which then tried four times to download it, unsuccessfully. The title would appear on the screen briefly, then disappear again. This is the original, persistent issue which I was enduring before the freeze-on-boot. The post-purchase page on Amazon reminded me to make sure my device was logged in and connected. Evidently Amazon isn't seeing it properly, either.
Since it's already dumped my downloaded eBooks, and isn't reading its own Settings, I probably don't have much to lose in doing a factory reset. What do you guys think?