LatinoReview is reporting that DC Entertainment is taking a page from The Wachowskis and Peter Jackson, and are looking to double-up on their upcoming shoot of Batman Vs Superman. Stating that Justice League will also shoot either at the same time, or right after BVS. The assumption is that Zack Snyder would indeed direct both projects in 2014.
This could explain all the chatter concerning The Flash, Green Lantern, Martian Manhunter and other JL members showing up in a Man of Steel sequel. LR believes their incorporation is meant to include a cliffhanger that will lead into Justice League.
It's actually an older concept which dates back to the 70's, originally Richard Donner shot both Superman and a bulk of Superman II before the release of the first film. Donner would eventually get replaced with Richard Lester, but the idea was to keep the cameras going and produce a film on a regular basis. The expected length of the saga was going to be ten Superman installments with Reeves but that ended with just four. This could be a cost effective way to produce more DC films, without each one costing the studio $200 million a pop.
They also confirm that Dwayne Johnson will be involved with Justice League, but his role is uncertain. There has been speculation that it could be the new Green Lantern or Doomsday, but more plausible characters could include Black Adam (back when a Shazam! movie was in the works), Lobo (which he was considered for previously) or JL member Cyborg (mentioned by Cavill and hinted to in MOS).