Thanks for reviving this thread, Pipeline. It’s amazing this goes back four years. Also you mention Melodyne 5, and I just upgraded.

If I’m reading your ideas correctly, you are proposing to create the jazz template by extracting chords from RealTracks. I wouldn’t have any trouble transcribing chords that are played but I’m not sure how that helps here. My assumption is that UserTracks must be created in a certain way for them to be useable by BIAB and I don’t know how to set that up with complex chords.
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BIAB 2021 Win Audiophile & Mac UltraPak. Software: Studio One 5 Pro, Adobe Audition CC, Notion 6; Win 10 64 Pro. Hardware: custom i7 16 Gb; Roland Integra-7, Presonus Studio 192, Presonus Faderport 8, Royer 121, Adam Sub8 & A5X monitors