Trying to get back (almost) to the thread, I have no knowledge of rearward-firing missiles, but the Soviets certainly had rearwards-firing gun pods on ground-attack Fitters and Floggers, and possibly Frogfoot in the 80s - and maybe even now. I think that they were supposed to offer a "spray" of 23mm(?) to the rear as the aircraft pulled away from its primary front-facing attack. Pretty inaccurate, but I suppose it would hope to keep troops on the ground heads down.