Well, Pete Drake appears on "I Live For You" and on an outtake from ATMP called "Pete Drake And His Talking Steel Guitar" from the bootleg album "Through Many Years". Pete Drake had worked alot with Bob Dylan before as well. The song features a Steel Guitar(with a talkbox!), which sounds alot like Slide Guitar, so Pete Drake could be a suspect.
You can hear Pete and George horsing around with it, even a short performance of "Danny Boy" and "Bridge Over Troubled Water". I should try to hunt up a outtake of "If Not For You", I am pretty sure John would be joking around during the down time like he loved to.