I live in Michigan. Perhaps a trip to the hardware store to purchase a double ended screw (stud) such that on side replaces the normal licence holding screw and the other end can support a wing nut. Then when the bicycle rack is in place the two wing nuts (License plates are held on by two fasteners to keep them horizontal.) can easily allow the transfer to a similar assembly on your bicycle rack. A small hole be drilled into the top center of the licence, to affix to a more visible location on your bicycle rack.