Well, one thing that I would assume is that, even if the locks fail, the restraints wouldn't just fly up forcibly out of the person's hands. Also most B&M's have those... pieces that stick out to your side on the seat. I don't think that the forces would be strong enough to toss the rider out as long as they had something to hold on to. The one thing to take into consideration is that, most people would freak out if their restraint flew up while they were on a rollercoaster, which may cause them to be too panicked (sp?) and scared to grab on to something. They may even.. flinch or spaz or something and throw themselves off balance.
Of course all of this stuff is kind of.. an EXTREME circumstance, but I'm sure B&M restraints have more than enough failsafes on them to prevent these situations from occuring.