My first reaction to this is: what the hell century are you living in? Almost literally every person at Comic-Con has a video camera with an internet connection in their pocket. Not only is every piece of footage going to be captured, but it's going to be disseminated online almost immediately, if not in real time. Even if you asked every person entering Hall H to sign some kind of non-disclosure agreement (which is not at all practical) at least one of the 6000 people in that room is going to break said agreement. And there's almost no way you could enforce any sort of punishment. Even if you could track the leaked footage to the indivdual before the panel ended, there's no way you could single them out in that crowd before the panel was done, and probably no way to catch them on the convention floor afterwards. Which means that the only punishment you could really inflict would be, at most, to somehow cancel their badge. But that would only work IF there were a substantial technological upgrade to the badges themselves and, even then, it would be a completely useless punishment for the last day of the show, where no one is coming back anyway.
Which is to say that every person going into any convention HAS to go in with the assumption that everything that happens at the show WILL be posted online.
And it will spread around online faster than it will at the convention itself! Do you recall that pen-stabbing incident from a few years ago? That happened in Hall H. Those of us sitting at home were seeing Tweets and photos about the incident from the people sitting right there in real time. The people at the back of the room, because they were looking for the panel to start and not staring at their phones, did not know what was going on. People on the other side of the planet were more knowledgeable about the events going on in Hall H than most of the people actually sitting in Hall H!
This is the 21st century.
It used to be that conventions were a gathering place where publishers could make major announcements. And what happened was that the handful of media outlets there would report on things, and we would all look to Comic Book Resources or The Beat to read up on all the news. But since everybody is reporting the news now via social media (including the publishers themselves!) the convention as a venue for making major announcements is diminished.
And with the convention's value in this regard diminished, there's no need for it to be a deadline to have promotional material completed. That's great if you want to show you awesome new trailer (for a movie, or a new graphic novel, or whatever) to fans at a convention, but if it's not ready to go by convention, then you shouldn't show your half-completed thing. Wait until it's done to a level you're satisfied with and THEN release it. Tell the fans, "Yeah, we wanted to have it ready for San Diego, but we need three more weeks to get it right." Not only to you not have to deal with crappy bootleg cell phone videos of your not-quite-ready material, but you're also not competing with every other thing that's coming from the same convention!
Look, I get that you want to give the people at the convention itself a positive and personal experience and, just by virtue of being at the event, are already primed to respond very positively to help generate the buzz you want your thing to have. But having to go back after the fact and issue take-down notices, and try to put a positive spin on why you're trying to put a lid on a project that's already been openly discussed online just makes you look bad and kills whatever buzz you were trying to generate in the first place.
The reason fans go to conventions is for the first-hand experience. Trying to place embargos on what they see/hear is not only a fool's game, but one that you will inevitably lose. You can try to play by 20th century rules if you like, but it's not going to work and not going to make you look good.