Maybe with the MultiRiffs, Biab could use another instance in the background (if necessary as it might be able to generate up one riff at a time using the one track ??) as RB does to generate generate up 7 choices for your selected bars/beats then it would only be a matter of copying the MultiRiff track section data (realtrack file, position, transpose, tempo etc..) then replace the existing track data @ the chosen bars/beats of the frozen track, then regenerate that track back to RAM using the frozen data plus the new data from the MultiRiff. If that riff is not suitable do the same with riff 2 and so on, if a suitable riff is not found the track will revert back to the original that Biab froze beforehand.

And/Or create a BiabVST that will do all this in a nice DAW smile

Biab Track Injector for Reaper
Free Biab Chord Picker Tool