Look, it's neither endorsed or suggested that you delibirately aim for the head. BUT should an opponent only stick out his/hear head then he or she is making that choice and knows the risks. If that person does not want BBs to strike the facial area (well, at least wants to minimize the risk of that...

As far as I know headshots are allowed if it's the only visible part of your opponent's body.
Always have been such at Swedish AS games...

Is it possible to book now, and pick them up at the game?