Hi Steve,
It is my view that in live or studio jazz no such thing as 100% improvisation is ever likely to occur.
Not sure if I understood you right, John, but what about jam sessions? You just say to your band mates that now we're playing Fly Me To The Moon in the key of Eb and you start playing. If you happen to know the people you're playing with well, you might just end up creating something beautiful. No rehearsals needed.
My two cents worth...