Maybe "side work" is what you are looking for? I can just now do a little of it but only on bluesy stuff. I use diatonic scales and the regular chord progressions that a guitar player uses. Can he "read" the guitar player's hand? Soft drones sometimes work too. Sounds like the trouble is that he can't hear what's in your head and I don't know how to tell you to get him too. Play what's in your head on a keyboard, or dum-ditty it for him?