Yes, they are actually somewhat common elsewhere in the world. There was a foreign website that listed several models for sale. I can try and find it..
If you are doing google searches you might do better to search for "shotgun moderator" as that's what most people in the world call them.
Tromix has one in the works for their SBS saiga 12's. Tony did a write up on saiga12.com forums about it. he said you "could" shoot it without ear protection, but it was still pretty loud. I think he is redesigning it now, as the baffles would get shreds of the plastic shot cup in it kinda like a cheese grater.
I downloaded the movie, and it seems as if he has it mounted on a shortened Remington 1100.
Although cool for the movies, I really think it would be impossible to make a shotgun THAT quiet. The action alone is pretty damn loud on an auto, and thats when you take away ALL of the sound from the shot.