The summer edition of Scoot Quarterly has a picture of a custom one wheel trailer on a sixties Vespa SS 180. It's a realy classy ordeal, with a connector for the brake/tail lights. It uses two pin joints so the trailer can move up and down (for when climbing hills) and side to side (for turning). I think this is how I'd do it too. I don't know that the ball joint would prevent the trailer from tipping over sideways. Dunno.