I'm thinking if it doesn't work, I can in the worst case copy some assembly from the working firmware, to maybe provide a usb device or light an led, and possibly bisect problems to troubleshoot. If I can keep it up. We'll see.

The unihertz thread shows ValdikSS got treble_experimentations to work from a release on January 18th, 2021. There's a strange inconsistency, unfortunately: the instructions say twrp can be used, but the twrp post is later that year. Still, it makes sense to try the last release prior to Jan 18, 2021.