Nope, rear firing weapons on 'mechs/tanks is completely ignored in BF/QS right now.
I think the major problem with including them is heat. The ability would rarely be used, but if available, would have to be included in heat calculations. That would really penalize most 'mechs carrying rear-firing weapons. Perhaps as an ability like FLK or IF, where you can't make a normal attack if using rear-facing weapons that turn.