I previously wrote that a likely candidate for the original microcontroller that this uses is an Elan Microcontrollers EMP series.
Good news, everyone!
You can buy them at LCSC:
They are still one-time programmable, but the datasheets have been translated into English, and they are 17 cents in singles. 11 cents in QTY 100.
It's a 32-bit Cortex M0, and I'm actually considering picking up a few now to do this project properly :)