Has it been done?
Small stage, intimate sound system. This is, shall we say, a "karaoke totem pole". Why let the singer have all the fun? Get an air drummer, air guitarist, air bass player, and make an entire air band! Depending on the number of volunteers, you could make a big band jazz ensemble, a blues set with harmonica and piano, even country lap guitars. The only person with equipment would be the one with the microphone, and they don't even have to sing (probably better if they don't, especially if they don't know the words!). You need NOTHING in the way of musical training, just pile on the musical emoting and you're ready to go.