Whilst doing the sound at Alcester & Arden several years ago, Dave did a special session with guests doing a singaround on stage, as if in a session. Nobody really knew what was going to happen, and the combination of artists was fairly 'unusual'. As I recall, Janet Russell, Harvey Andrews, Jim Couza, Pete Morton? - may have been another. Janet started it off, and they then went round bouncing off what the previous person had done. On some, they played/sang together. It was totally impromptu.
I have to say, it was one of the most magical and enjoyable concerts I have ever sat through.
On the same year was the Copper Family in conversation. Bob was still alive then, and all the generations were there. They sat behind a long table, sort of committee style, talked to the audience and sang songs in rotation. I suspect this was a bit more contrived, but a very special experience nonetheless.

Do it Tim. If you get the right people, it can work brilliantly. But, someone needs to hold it all together, otherwise it could become a bit of a shambles. You can't really have breaks when performing on a stage.