Well, Mike gets solo slots. They will extend the solo section to like 20 times the length and he'll just go crazy for 3 or 4 minutes. They are improv by their very nature because the solo was never that long on the record.
And yeah, I agree Dean could play with that sort of feel if that were the sort of thing STP do, but it isn't. He occasionally loosens up in the jam bits between songs, but I have never seen them even attempt a Hendrix style epic guitar solo.