UserTracks were a great idea I remember a post back in 2013 Home Made Flamenco Realtracks the user chorded out the RealTrack audio files then swap them with his playing.
I don't think enough development went into UserTracks as there are still a lot of outstanding issues that never really got resolved.
ReaTrak was made to integrate Biab with Reaper and now we have the Biab Plugin that's great and seems to have a lot of ongoing development !!
Reaper was used as it has API's to give control of all of Reaper's functions (Studio One has some control with scripting but nothing like Reaper as for other DAW's nothing really).
Reaper is crossplatform Win/Lin/Mac and is free to try n try n try...

With ReaTrak you can import your existing UserTracks to work directly in Reaper by saving the UserTrack chords to MusicXML and dragging the xml into Reaper then select the item > "MusicXML Chords" then drag the audio track in, then save the regions to csv (Export/Renumber Chords) and the audio or midi with the same name:
Guitar, Nylon, Fingerpicking Pop Ev8 4-4 80bpm A-F#m.csv
Guitar, Nylon, Fingerpicking Pop Ev8 4-4 80bpm A-F#m.wav
in a folder Guitar, Nylon, Fingerpicking Pop Ev8 4-4 80bpm
so you will have time signature, tempo and key in the filename,
"Drums" for drum file and "Scale" A Major or A minor etc.. that will play over any chords in that scale.

The audio file can also be mid, mid & audio, mp3, wma, m4a, aiff, wav, multichannel wav, multichannel opus, ogg, multichannel ogg etc..
You can add shots & holds to your UserTracks and add them the same way to a folder
Guitar, Nylon, Holds 4-4 100bpm
Guitar, Nylon, Holds 4-4 100bpm.csv
Guitar, Nylon, Holds 4-4 100bpm.wav
you can look at the sample/testing tracks to see how they are made test_files
Include the *.reapeaks file (and reapindex for mp3) if you drag the audio file from the folder into Reaper one time only and in will then create these files to show the waveform.
You can also create a lot of different intros/pickup notes in the 2 bar count-in, to add more count-ins just add a 2 bar region anywhere and set it to "Count-in" same with endings these can be 4 bars or more.
ReaTrak will also allow you to generate up RealDrums directly in Reaper giving more control of volume levels, shots, holds, pushes, endings and intros.
Along with be able to use multiple Biab Plugins (a Biab Plugin on each track) this will give you a lot more control of Biab tracks and usertracks, you can also import the created usertrack back into Biab in the Audio track.

Hope this helps to give the results you need smile
ReaTrak Integrate Biab & Plugin in Reaper
Free Biab & BiabVST Chord Picker Tool