Remember you have to be a headliner or pretty close to get a long enough slot to do more than one set. Some artists do acoustic sets to mix it up some in longer shows but not a 45 minute show. So really it's acoustic songs, which is even harder to predict.
It's not highly doubtful. It simply won't happen unless there's a power outage. I saw a couple of those videos though, awesome.
I went to a Porcupine Tree concert one Friday the 13th and they had audio problems, and because of that I was able to see Steven Wilson perform his cover of Alanis Morisette's "Thank You" on acoustic guitar. That was a great moment.
Ron Paul 2012!
Death Cab for Cutie/Magik Magik Orchestra
some other bands