Okay that's all coming back to me now.

If I take bb\Data\CustomMIDIPResets.txt and add the rest of the instruments from all the other midi .STY to it then make it into a proper csv that can be sorted in the floating picker/browser.
It would preview the the midi by using it's patch with the default synth and playing back the midi contained in the .STY as a 4 Bar Preview.
So if it's what you want it would then generate up that instrument in the chosen track.

The first instrument in the CustomMIDIPResets.txt is

Synth, Alternative Rock A-B Ev 170 Any$altrock1$strings$103

Memo: Alternative rock even 8ths feel. 5 instruments: acoustic guitar, bass, drums & high synth at A. Add distorted electric guitar at B. T=150-190 -Examples: How You Remind Me, Semicharmed Life, Genre "Intensity" Score=70/100
ALTROCK1.STY. Alternative Rock 1 Style
All MIDI. Quintet. Tempo=170 (150-190)
Instruments: M-Finger Electric Bass, M-Distorted Rock Guit, M-Acoustic Guitar, M-Echo Sweep
M9 Room Drum Kit
M34: Finger Electric Bass
M31: Distorted Rock Guitar
M26: Acoustic Guitar
M103: Echo Sweep

I'm thinking it would be easier to batch extract the instruments from the midi STY to the CustomMIDIPResets.txt but maybe these have been manually named ? You wouldn't want to go through them all manually.

Trombone, Simple Root Ev 100 Any$Brass_44$guitar$58

Memo: This Style will emulate a brass quintet playing in 4/4 time. T=50-200 -Examples: Fanfares, Rule Britannia,, Genre "Intensity" Score=10/100
BRASS_44.STY. Simple Brass Quintet 4/4 Style
All MIDI. Duo. Tempo=120 (60-180)
Instruments: M-Trumpet, M-Trombone
M57: Trumpet
M58: Trombone

There are tools that preview instruments in Yamaha prs sty styles, I can't remember if JJazzLab will do that and import the separate tracks from different styles.

I just had a look a the keyboard styles in JJazzLab and it's much the same, it has the style name and basic instrument names, they don't use 8 character names anymore for the keyboard styles.
Modern Pop Ballad.T173.prs
Mod Orch Ballad.T173.STY
Big Band Shuffle.T157.STY

After going all through this how it works now with the Custom Midi Dialog, it just needs all to be in one floating picker that you browse the styles and each style will display the contained tracks that you can select and preview without going in and out of jail dialogs, if it's what you like it will generate that instrument into the chosen track and allow you to choose another track and another style instrument without going in/out of jails. A floating compact trimmed down version of the current style picker that just shows midi styles.