I just did some more digging and experimenting, because it just seemed to me that it would be so weird if -- as I recall seeing a few times in the manual -- BIAB does let you record audio live, to be part of a BIAB song; but then would not let you add a pre-recorded audio file for the same purpose. After all, user-created audio is user-created audio, right?

My conclusion is that I have to respectfully disagree with y'all -- what I'm trying to do can be done after all.

Here's how. In my case, import the midi file. Then, instead of going to File/Open Audio, go to Audio/Import Audio. That's it.

Now the Audio will accompany whatever BIAB generates off the midi file's chords, given the chosen style. The levels and panning of the audio and the BIAB-generated accompaniment parts can then be adjusted in the mixer.

All's well that end's well I reckon.