I am wondering...
because of style variation segments it will be challenging to just superimpose Jjazzlab arrangement to the one in BIAB.

I imagine, an intuitive "third party" (BIAB+Jjazlab) integration can look like this:
Jjazlab as a VST in BIAB, duplicating whole chord grid from BIAB, following all changes being made and tempo map. Outputting sound of whole arrangement from BIAB+Jjazlab simultaneously.

I am not sure about the part where VST can sync (duplicate) chord changes as they being written in BIAB, but if it is possible....

Maybe it worth the shot of asking Jerome to develop such VST while he is full of energy? (packing what he has in VST shell and making proper connections?) As I imagine, if it is possible, this VST could be used in such manner in any DAW that has chord track or similar.

Pipeline, if you are reading this, do you think such workflow is possible? If it is not realistic for technical reasons, maybe it is better to leave it as imaginary concept and not bother Jerome?

Thanks.