Including a reference to the iconic Watchtower, the eventual base of the DC super-team, would be one simple and sure fire way to foreshadow the upcoming Justice League movie, without even having to fully reference any one specific character. Youve seen the Man of Steel trailer, right? That final shot, in which Superman leaves Earths atmosphere to head into outer-space; theoretically, the very next shot could reveal the Watchtower, as yet empty and inoperable, but looming there just waiting to be inhabited by DCs mightiest superheroes. Its a simple nod, which could be added in during post-production, and wouldnt even necessarily have to take up much, if any, space in the script, yet itd be noticed and appreciated by almost everyone whos had exposure to the Justice League in the past.