I've also been pondering a powered cart. For moving a standard trailer, perhaps mounting a small engine with a centrifugal clutch on a Seitech or Trailex trailer mover would work.
http://www.castlecraft.com/trailer_mover.htm

Or perhaps a submersible electric motor driving one of the wheels on a Seitech #15 dolly. http://www.castlecraft.com/dolly_models.htm