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:

https://lcsc.com/product-detail/ELAN_EM78P173NSO14J_C108337.html

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 :)