cxp,
Thanks but... The request is to get away from 90s workflow or/and other tedious and bizarre workarounds.
What I am asking was done by quite a few titles in recent 10 years. Some are even free or very low priced. The problem is integration with BIAB.

Particularly what I am asking for is to: put my own MIDI loops in a folder, lets call it "User MIDI" and so they show up in MIDI trk tab in MTP Library and act same way as PGM MIDI snippets > reinterpret phrases according to chord sheet same way as it is done with MIDI trks. Simple as that. No need to reinvent the wheel.

That was actually my first major disappointment when I started using BIAB - Styles that don't have proper parts for Intros / Breaks & Fills. Since it would be extremely challenging to make the parts out of RTs of existing library, MIDI loops/phrases that are accessible the easy way and a combination of "playable RT's" or decent VSTi would make a viable alternative.