I think like SinnohTrainer said, the fact that there's in-game effects resulting from getting these Pokémon into Black or White would be a big factor in holding the event outside of Japan (though of course their main purpose is to promote the movie...). Personally I also think it'd be a guaranteed Toys "R" Us promotion in the likely event it were to be held live, because there seemed to be a trend during the latter half of the DP anime saga of movies where there would be the starring Pokémon of the particular movie as the main video game distribution, coupled with a supporting distribution (containing supporting Pokémon from the movie), usually held around a month or two before or after. In the case of Movie 11, it was Shaymin followed a month later (at least in the U.S.) by Regigigas, and in Movie 12 it was Arceus followed by the Pikachu-Colored Pichu two months later (though the Pichu in particular was a stepping stone to the Spiky-Eared Pichu, the real plot supporter, and wasn't held at TRU). In Japan it's already been signified that Movie 13 follows a similar method, with a main Celebi event, and the supporting Shiny Beast event. Of course, it'd be foolish to go only off of observed trends, but what I really think it boils down to is the fact that these Pokémon are promoting a film, giving them higher priority from officials. That's the same reason you don't see specific events like the Pokémon Center Tropius or Whiscash leave Japan.
But assuming it were to be held, I agree that it's going to be at least a four-month wait...