Ah, I had to set ep128emu to use a disk-based config, then the MBP.COM-player worked just fine!
And as you said, these .mbp tunes turned out to be precompiled for 0x4000, so it was just a matter of dropping them into my program and adding an init-call to 0x4000 and then 0x4003 every frame, and now I have some music to test with!
So now I have a lot of options:
1) Use one of the exisiting MBP-tunes in my demo.
2) Have one of the musicians I know learn Music Box and create a new MBP-tune in it.
3) Have them make a AY/YM song (for example in PT3) and use that via emulation. Problem here is if IRQ for envelope-emulation is needed, then it would steal CPU time from effects which is not good at all, and also it takes away IRQ from my own usage.
If it ends up in option 3, is your PT3player-source (which I guess has the Dave-emulation calls in place) available Zozosoft?
Otherwise I can probably adapt it from the ZX Spectrum code I have and insert IstvanVs emulation in the right place, but it can't hurt to ask before doing the extra job?