Definitely +1000 !!!
This is a really missed feature of BiaB. Presumably it cannot be incredibly difficult to achieve.
Another +1000000 !
Likewise I doubt I will ever use RB in its present state and this would be a superb feature to add to BIAB.
I can see how it might work for the User - "highlight" a track, maybe using the current "freeze" button with dual function, click on the first bar from where you want the re-gen and hold and drag along the bars to the last bar you want re-genning. That area might then turn say Grey and Re-gen will only re-gen that portion of that track until you un-highlight that track. All the other tracks would effectively be frozen until that partial re-gen function was cleared.
Go on, PG - you know you want to!
cheers
ian