2. Super-City References
To reference a larger DC continuity, its not even actually necessary to include character cameos, or even particular references to anyone directly. If someone mentioned Coast City, I would immediately know that thats the fictional home of Green Lantern Hal Jordan. Similarly, if someone dropped Central City into conversation, Id know that thats where The Flash Barry Allen hails from. Supermans alter-ego Clark Kent works at a newspaper, right? I mean, its not a massive stretch to say that this is more than likely going to feature in some way in The Man of Steel. You couldnt really ask for a more convenient way to subtly establish a larger world without forcing it in; someone is sent to Coast City to report on this, someone goes to Central City to investigate that. This would be a relatively unobtrusive way to reference that larger DC continuity, and while itd admittedly stand out much less to the non-comic book fan than a direct character cameo, itd still help Warner Bros. to create that sense of a wider Justice League continuity outside of Man of Steel.