A quick excuse the pun - reference to mild-mannered Barry Allen (the Silver Age Flash; another important, well-loved member of the League) would be easy enough to squeeze into the Man of Steel at some point, without too much disruption to an already completed screenplay. Were not necessarily talking The Flash either per se, maybe the Allen were introduced to in the Man of Steel could exist before he was doused by electrified chemicals and given his super-speed. He could still be studying Forensic Science at college, and visits metropolis as part of a field trip, but gets caught up in the carnage thatll inevitably ensue when Zod touches down. The problem of casting could be easily made inconsequential by making whoever played him in the Man of Steel a younger version of who hell be in the Justice League movie, and his inclusion would certainly allude to something greater behind the fabric of the film world.