You could do all that and maybe after many hours it might work, or you could also solve this in 5 minutes by just plug in the magic keyboard to the computer, stick it in a box with a cut out for the touchid sensor, and just stick your mechanical keyboard on top of the box.

“requires a donor [Apple Magic] keyboard” is not the same as a third party making a Touch ID-compatible sensor. That’s Apple making a Touch ID-compatible sensor and a third party taking it out and putting it in another keyboard.

This does not qualify as a third party Touch ID compatible keyboard.

Yeah, but it’s a third party mechanical keyboard with a Touch ID sensor.

I suppose if one came across a pile of old / broken Magic keyboards, they could extract a bunch of Touch ID sensors and sell those to a keyboard manufacturer (or in their own keyboard). This does not scale, obviously, but including it as an option could work.