I finally tracked down the Rolling Stones Rock and Roll Circus - it was filmed in 1968, released to the public in 1996, and it's been on YouTube for at least seven years - but I didn't think to track it down.
I'm sorry it took so long because Lennon does "Yer Blues" with a bunch of other famous musicians, not in the Beatles, and it's great.
I've never heard anybody else remark on this, and maybe it's too obvious to point it out, but I think the entire Get Back project was strongly influenced by Rock and Roll Circus, with Lennon performing for the first time in public for an audience that wasn't all screaming adolescents.
You can see the audience at the end of this performance, and while enthusiastic, it is much smaller and more subdued than anything the Beatles had experienced in the previous 5 years. Here is a comparable kind of venue three years earlier. It's amazing how different Lennon looks in only three years.
You can tell Lennon was thinking about it during Get Back because he mentions "our hosts the Rolling Stones" or some variation several times throughout the filming.
