Another update. Finally, a sample based AY engine! Reverse-engineered the code from Sample Tracker 2.1, and as it works with 48K, it was possible to fit it into the *.AY file container (although I've got ideas on how to go 128K and beyond).
I had to modify Game_Music_Emu in order to make it work, as it didn't fully support simplified AY ports decoding.
The engine also supports Covox, Stereo Covox, and SounDrive, I kept it in Z80 code, but GME does not support these devices, and it seemed to be kinda difficult to add them in, so for now these are not audible in the tracker, but still can be exported as code. This may change in the future.
The engine is already bundled with the tracker. Here is the original ST2.1 reversed code without much modifications, just in case anyone wants to look at it.