Oh boy, this looks bad... Why couldn't I have waited... I rebooted in fastboot mode, and used yifan's fastboot tool for Mac OS X with the "sudo fastboot erase data" command as you suggested, which returned success, followed by "fastboot setvar bootmode main". Hard reset, it blinked twice, and now a new phenomenon, an all-black screen. Yikes. I left it overnight and no change. Additional hard resets cause 2 flashes and then return to black, and it does not mount even an inaccessible USB drive. It appears to not be in fastboot mode, since the tool can't communicate with it anymore. I *can* get it into HID mode, and the MfgTool can communicate the Diags image and cause a reboot, but again just to a black screen.
One more thing that's relevant is that I cancelled halfway through the first time I tried "fastboot erase data", because it was taking minutes (the second, successful time took over 5 minutes). I hope that doesn't mean I've corrupted something.
Again, I'm so appreciative of your hard work geekmaster, and I'm willing to accept that this time I've messed it up for good, maybe in a unique way. But if you have any ideas for things I can try, I'd love to hear them.