Another performer did ask me to do back him up for one of his songs on mandolin, just making up a counterpoint part to his three-chord song, though, and I did, and that actually worked pretty well. Sound guy in particular liked it. When I'm second banana on stage I'm a lot calmer; I just watched the guy for his changes and made stuff up as I went along, and that was easy and apparently popular. He videos his performances and has a myspace page, so I'll post a pointer when he sends it to me. (Assuming he does. He said he would.)
And now lo I am wiped.
eta: Decided to listen to the recording before going to bed anyway. 1. Pitch control much better. The a cappella opening to "Thought You Knew" still isn't quite hitting, but was better. 2. Tone control extremely spotty... tho' I don't sound as entirely bad as I'd expected I'd might, for once. Some quite bad spots, others not so bad. Clearly need much more work at getting a grip on that; I can tell that while I was occasionally not cutting off my sinuses, most of the time I was, and that's fail. 3. Too damn fast. Again. Mandolin work sounded better than I'd expected it would, and would probably have been okay if I hadn't been too damn fast.