That's spot on my opinion of any sort of BB blocking device. The trouble with airsoft across the board is we're trying to emulate a firearm with a BB gun in essence and worse one that is safe to shoot at other people. That means the ballistics always suck, and a single BB can never be as effective as we'd like. So as a result along with all the other compromises in our chosen hobby any sort of barrier is a bigger problem than it would otherwise be. So plywood becomes a complete blockade to BB's, etc etc. Riot shields could work, but like mentioned it's just far too easy to abuse, unless I had a scenario that called for their use in some sort of scripted event scenario I wouldn't allow their use here. That said it's always going to be up to the people running the event, but you've gotta keep in mind their primary goal is that people that attend the event have fun, and enjoy themselves. Shooting a riot shield all day without getting the person out isn't often the "fun" they are looking to have;)
I'm not suggesting you're planning on using it like that, but it's just the reality of our hobby as a whole it's all the compromises we make to play.
Luke