I'm reading a Stevie Ray book right now and came across a funny story of a 15 or 16 year old SRV asking if he could come on stage and jam with ZZ TOP at some club in Austin in teh early 70s. Billy invited him up to play one song with them.
The story finished with Billy Gibbons making a new rule after that show that ZZ TOP was not going to jam with other musicians, especially kids.