I try and work out ways to do everything and stay in Biab, I requested being able to drag n drop audio directly into the BB Audio track, you can do that now but it still needs a bit of improvement, and hopefully we will see the same soon with drag n drop midi into the piano roll.
Every part of the BB program relies on the standard track set there is, so adding more tracks would be a major all over change.
But if they could add some Aux Audio/Midi Tracks that would help, if the Audio tracks could be RealTracks in RAM or not I don't know if there would be any issues with more than 7 tracks in RAM maybe not ? if so they could be just local wav files same as the BB Audio and editable in the Audio Edit windows. But either way I think it would be good to have more AUX Audio Tracks as well.

Would that and bar to bar re-gen be something usable helping you to stay within Biab ????
Or would you like to see....????

Biab Track Injector for Reaper
Free Biab Chord Picker Tool