I think I can almost relate this scat-singing thing: you are in a jazz hall, probably somewhat under the influence of 'something' and the instrumental music is so loud, its the only thing you can hear--its so loud you can't think. You start to utter sounds, but, these sounds are within a certain key. So, in my opinion (as a former recreational scat singer) you don't have control over what comes out--other than keeping tones in key. You are 'overcome' by the spirit of the music, or one wouldn't sit on the sidelines and scat sing anyway.

I don't see it as any different than improvisational music, or dancing for that matter. You don't know what you are going to do until a millisecond before it comes out, and if you get profficient enough, you close your eyes and it just happens.....

Check out some of the jazz and reggae on BETJazz that they show from the Carribean--those guys go into trance for hours.