Not a "freight dog" joke per se, but one of my friends who flew pax in the 727 back in the 80s would have the FEs remove all the light bulbs but one from the auxiliary warning panel (bottom right on the FE's main panel) except one. They would then wait for the stewardess to arrive with their meals and would tell her that pushing the test button below those lights would show how she likes it in bed. The unwitting stew would play along, only to see "AFT ENTRY" light up in orange.