Seems that two popular items in BIAB community are Scaler and EZkeys. These plugins, along many others have MIDI out option and there are quite a few MIDI plugins that do not have sound source of their own and rely on secondary sound producing Synth / Instrument.

Benefits of routing MIDI is that you can use EZkeys to play completely different instruments. The ones that you can't buy from Tunetrack. For example strings or wind. And for the arpeggio or other phrase based instruments that don't have sound source have them play along side of your BIAB arrangement with Chord Track or by programming their internal sequencer.

My preferred way in working with BIAB is to try out different RTs with sounds, grooves etc from other 3rd party tools that I have, before committing exporting project to DAW, as I change things around quite a bit. So kindly don't suggest doing what I am requesting in DAW smile

So the request is:
Possibility to route MIDI Out from one VST to second VSTi Midi In within BIAB.

To my surprise, BIAB doesn't have this option.