Deak Harp in his one-man band incarnation, with diddley-bow, is certainly the closest I've heard. He does a one-chord song and goes for 10-15 minutes. He did it at HCH 2012 on the gazebo down the hill.
I had thought of mentioning Deak based on his long 1 chord blues grooves and talking style of vocals. When I saw the Sonny Terry reference I decided I had better think it over before posting.
I think that Deak creates a similar driving groove to that of John Lee Hooker, with a similar vocal style. ----------
