Okay, I noticed the back of the device was excessively hot, so I set it on an extra heat sink that I had lying around. I unplugged the AC power, waited a few minutes with the device on the heatsink, then tried again, and I got that the battery was low. I plugged it back into the AC charger, then tried again, and...
SUCCESS
It booted right up. So, we may have a heat sensor while in fastboot... I'm not sure. When I restored, I did boot.img, system.img, userdata.img, usercache.img. I left out recovery as it was never modified.
So for anyone else with this problem, unplug it, let it sit for a while and cool off, then plug it back into AC and try again.
I *think*, and I'm not sure of this at all, that battery drains while in fastboot mode, even connected to a computer (it's hot because the device is working hard/inefficiently/draining battery). So don't leave it there for long, do what you need to do and reboot.
Maybe the old trick about using a "factory cable" might fix that, but it's all just conjecture.