And one other thing...

Lots of pieces of music have their own particular individual timings or "groves" if you will. The non-standard timings may even shift repeatedly as you go through the song. If you bring in a midi and isolate the melody, it's not at all uncommon that the timing for any of the existing styles doesn't match the individual song. This can be "fixed" through the use of the quantize feature which then allows selecting an appropriate style, but only at the cost of losing a significant amount of character of the original melody.

What one would then like to do is freeze the accompaniment and go back to the timing of the original melody carrying the relative timing of the accompaniment with you.

It is possible to adjust the timing of the melody and accompaniment outside of BiaB to recover the original grove, with a greater or lesser difficulty depending on the tools one has available.

But... It would be very cool not to have to leave BiaB to do this, and not everyone is going to have the appropriate tools and know-how. BiaB is at it's best when it makes things easy.

I tend to suspect that, since BiaB can adjust melodies from swing to straight to waltz and of course do a variable quantize, that much of the machinery you would need to accomplish this is already there inside BiaB.

If you really wanted to go over the top, you could forget freezing the accompaniment and every time you generated a new accompaniment you could go through the quantitize-generate accompaniment-unquantitize cycle and call it something like intelligent styles (that adjust to your melody) and turn it on or off with a single check box.

Doing this with RealTrack styles would add a whole nother level of difficulty, though with time stretching capabilities and judicious substitution it might be possible. But it's the midi version that would be the most useful.