He did a short "Barbara Ann" with Brian and Mike in 2012 on his show.
And during that segment, he seemed so awe-inspired by the idea of his voice being part of a three-part harmony. I'm totally not big on feeling like I need to get on stage with these guys, and "Barbara Ann" isn't exactly my favorite, but it would be fun to sing a three-part harmony with those guys just for a few seconds.