I Have an idea:

In the original campaign scrips, printing certain things will make the game exit and go to another mode. For instance, at the end of the last level, there's the command "Print("creadits")" -something like that. And it causes the game to go into the credits. Something similar happens for the beggining bungie movie and the ui wait screen movie. It would be great to have something where you can build these functions in OS to play movies.