Add ABLETON LINK support - including the start-stop sync API in the latest version.

This would greatly enhance being able to work in - say an iOS app or Ableton while syncing up with backing parts in BIAB.

Sure. This is a specific request for Ableton but it applies in general to any DAW. I think what you might be requesting is reflected in other wishes to make BIAB operate as a VST, as one possible solution.

Yes a +1000000 for BIAB as VST

It would be the biggest (in size) VST ever, by orders of magnitude. But it would be terrific: groundbreaking.

Actually its not. Ableton-Link is a protocol Ableton made free to any developer - which allows apps to be synchronised by tempo - over Wi Fi or USB . Its used heavily in iOS apps and many PC/Mac apps also support it.

Here is some blurb from Ableton themselves;
"Ableton Link is a technology that synchronizes musical beat, tempo, phase, and start/stop commands across multiple applications running on one or more devices. Applications on devices connected to a local network discover each other automatically and form a musical session in which each participant can perform independently: anyone can start or stop while still staying in time. Anyone can change the tempo, the others will follow. Anyone can join or leave without disrupting the session."

read more here:

https://ableton.github.io/link/

Thanks for the clarification about that. I read the description.

How widely has it been adopted?

One way round this is it could be a kind of BIAB "Client" plugin - with the normal BIAB app acting as a "SERVER".

In fact there are many reasons why having a server-client architecture would be useful for pro composers etc.