To do it all in Biab they could have an 'Extra Tracks' button that opens a separate DAW style tracks window that you can drag n drop both Audio & Midi files in, as you get many user asking for extra audio tracks.
This could have it's own separate mixer and on the BB Mixer have a Master slider for it, or maybe have an arrow at the bottom of the BB mixer that expands it to show all the current Extra Tracks.
This way you could also drag a RealTrack/Drum into the Extra Tracks window, you could also Multichannel RealDrums into the Extra Tracks window giving you a lot more control over the Drums.
But as said before they have the Plugin now and that may well take off in a big way allowing you to do it all without leaving the DAW, as it's more of a Revolution than an Evolution of Biab (BiaDAW).
Maybe Biab will stay how it is for auto accompaniment, practice, learning, song writing etc...
Instead of bringing the DAW to Biab they have brought Biab to the DAW.
And ReWire added to Biab will give you all those Extra Tracks and could be easier to implement.
But you never know, I've lost count of how many times over the years "Ya Flogging a Dead Horse give it up, Ya dreaming, give up these silly notions, that won't work...." so were they pipedreams that pipeline had ? well here we are with vst sync, 64-bit (& still have a 32-bit) and a BiabVST !!!

Biab Track Injector for Reaper
Free Biab Chord Picker Tool