Both good ideas and I'll try both.

I'm going to go out on a limb and propose something that might tail into this. One of the other limiting factors in BIAB is that you only have a limited set of 'players' in the band: Bass, Drums, Piano, Guitar, Strings, and Melody. Yet there are times when I'd like to have the choice of additional players without having to fudge around like Aubrey and I have, creating multiple files, etc.

What I would suggest (for now) is something like a workbook grid that pops up that allows us to enter something like:
---From-- ----To---
Instr Bar Beat Bar Beat Relative?
365 1 1 1 4 Y
365 12 2 13 3 Y

So, for example, the first line would mean 'play the "Guitar, Acoustic, Fingerpicking, EV 085" from Bar 1 beat 1 through bar 1 beat 4 and from bar 12 beat 2 through bar 13 beat 3, or using time stamps would work as well.
Then BIAB would do it's best to only play during those bars/beats ONLY without having to mute other bars, etc, and would assume in it's algorithm it should start and stop elegantly for this time of instrument.

The "Relative" column with value of 'Y' would mean 'shift the bar and beat markers according to edits done by the user.'

Something like that! :-) Cludgey but it might work...