I've also found it curious how Yamaha chose to code the styles and style time signatures. It definitely doesn't match conventional music theory, but yeah just the sound of the style communicates a lot more about the feel of the music.
I have recently used a 6/8-ish style, PopWaltz, that is 6 main beats long but can repeat every three. It is coded in 3/4 instead of 4/4. But this style is actually an 18/8 time signature style which goes a full 18/8 bar but can do 9/8 bars too (half bars) even on my S950 without the nifty half-bar fill feature. (18/8 is 6/8 where each beat is a triplet, so it is a double compound meter, and it is often counted on a metronome as 6 beats long)
I expect one could create a 6/8 style that would operate like PopWaltz and allow 3/8 and 9/8 measures. Of course I'd ask what you mean by having a 9/8 measure inserted into a 6/8 piece. I'd think it would usually be a 6/8 bar extended by another 3/8 half bar. With 6/8 usually the emphasis is on beats 1 and 4, whereas in 9/8 the emphasis is usually on beat 7 or maybe beats 1 and 7.
The existing 6/8 styles I've used on my S950 are actually 12/8, as they're coded in 4/4. You could edit one of those styles to code it in 2/4, making it 6/8, but that still doesn't give you your 3/8 bars. For that I'd expect the style would have to be coded in 3/4 instead, which would require quite a bit of work in a MIDI editor to convert the triplets to straight quarter notes, but if you did that it should give you the flexibility you're needing.
- Greg