Nevermind, it's sorted now.
Using RK27DM, I was repeatedly able to gain access to the firmware partition. After updating to the M29 firmware, I performed the rkusb.tag trick, then manually updated to
this firmware, intended for a device named Kingzon, and lo and behold, the device was able to reboot and display graphics. Vertically flipped graphics (upside-down and backwards), but graphics nontheless. Taking this as a good sign, I then updated to my original backup, and now the device works fine. Or as well as it did before I lobotomized it.