So long as the mag is outside the borders of the PRNJ I don't see any problems with NJ law enforcement. It's not as though NJ can arrest you for keeping an item within PA state lines that is prohibited in New Jersey. It would be analogous to me visiting NJ and being arrested and charged with illegal magazine possession since I own banana magazines which aren't even in the state of NJ.
However, PA may have a statute prohibiting possession of hi-cap mags by persons from out of state. I doubt it though.