I don't believe the idea was ever tried on a single-seat fighter. Multi-crew aircraft might get a 'scare gun' even if not originally fitted with such (as I mention in a previous answer on turret gunner effectiveness), but rigging something up in a single seat fighter would have been more complex, adding unnecessary weight and adversely affecting performance, and been all but useless. Surprise was the dominant factor in air combat - according to Mike Spick's Allied Fighter Aces of World War II "... in four out of five air combat victories the victor achieved a position of advantage before the victim became aware that he was under attack". Rearward visibility was always an issue, just seeing an enemy aircraft difficult enough, let alone trying to aim in any way using mirrors or similar.

Some Luftwaffe bombers used more extreme 'scare' weapons - rear facing flamethrowers, but these were limited to a small number of trial installations (a couple of Junkers 88s that actually flew on operations, and a Dornier 217 that didn't; "Perhaps not unexpectedly, the Dornier involved in the ground tests caught fire and it is not known if any air tests took place"- Robert Forsyth, Luftwaffe Special Weapons 1942-45).