"Third Party" is what it needs, third party RealStyles, RealDrums and because you don't have third party RealTracks we have UserTracks, sure as I said they still need fixing but you need the popularity for this to happen. There are third party EZkey/Drums & Bass libraries, third party Kontakt libraries.
"But this approach requires finding music producers who are willing/able to learn to use BIAB/UserTracks. "
The first session player I contacted about this said yes so is it worth contacting more ?
I bet ya if I contacted drummers or drum sites now and I gave them all the info I made for pg users here
https://www.realband.org/tutorial-drums1.html
https://www.realband.org/tutorial-drums2.html
they would be in on it.
I can do this now and I'm sure I would get a lot of others interested and doing it, but I'm a bit worn out these days from constant giving out frown So that's why I'm trying to encourage other here to do this.

Using "Styles", a producer way back told me that all Biab songs sound the same, I think this is because they use a style out of the box add vocals and that's it. I work with musicians that have done this, I thought that sounds good but then you hear the exact same style someone else has used for their song, so using out of the box styles I don't think is good, I think with all the RT/RD you should make up your own unique style for you own songs.
The poster said about using EZkeys and UJAM etc.. this would make it more unique.
What about a Dice that will create up a unique style from chosen range of inputs ?
Generate Style > Play > don't like > Generate Style > Play ....

I posted this elsewhere about how EZBass/Drummer has a search for same phrase and tap2find,
I said I could generate up all RealDrumCharts (midi) and put them into EZDrummer and I would instantly find the RealDrum I need same with all the RealBassCharts.
The same can be done with other instruments RealCharts.

Biab Track Injector for Reaper
Free Biab Chord Picker Tool