Charlie,
"for us to add solos or styles or whatever."

In my understanding this is a very clear general request, not related to a specific style or specific project.(or a workaround*)
BIAB is not a DAW, so sticking an audio file in it that will not respond to further chord changes will not solve request.
More available "normal" tracks in the mixer for RT's or MIDI will. 16 is a good number that most arrangers had in place for decades.

P.S. if it was me, I would block the word "realband" from the BIAB wishlist forum section... I believe here is where people express their wishes for BIAB not realband.