I'm not saying you can't do it, I'm just saying it's not convenient enough for most of us. I have folders with several hundred Biab songs in each one, all kinds of different styles including tons of PG's demos that I still haven't had time to audition yet. I want to click on a song and simply hear it immediately, no messing around required. A dedicated GM synth does that just fine. Virtually all Biab files and probably 90% of midi files you find on the web are GM so why reinvent the wheel? As I mentioned I use Sample Tank all the time and tried their Omnisynth Gm soundbank and you still have to go into the ST window and configure the instruments and channels for each song unless you want to either bypass the Biab style instruments or only use the same style over and over. And, some styles definitely do have patch changes in them but I can't think of one right now. The Ketron SD2 and my Roland Sonic Cell sound great just using their GM banks and both have patch maps to access their higher banks.
If there's a specific tune I decide to get into and work on, I will put that into Real Band, why mess around with all this in Biab when RB already has 16 midi ports, 48 tracks, the same chord grid as Biab, Real Tracks/Drums, audio/midi recording, tons of editing, the whole thing?
Without studying all the details of the lastest softsynths like that Hypersonic2, my impression is they're all disc streaming and that's the kill with Biab. Whatever synth is being used with Biab has to load into ram afaik but maybe some of these new ones will work, don't know but frankly RB is better anyway for someone like you who seems pretty advanced with this stuff. Most of us will get the tune started in Biab using a GM synth just to audition it and if we want to get serious with it, move it over to Sonar, RB, Reaper, whatever.

Bob

Biab/RB latest build, Win 11 Pro, Ryzen 5 5600 G, 512 Gig SSD, 16 Gigs Ram, Steinberg UR22 MkII, Roland Sonic Cell, Kurzweil PC3, Hammond SK1, Korg PA3XPro, Garritan JABB, Hypercanvas, Sampletank 3, more.