I think when it comes down to it, short of writing your own codec or something, if a user wants to view the videos, they'll do it. For the most part though, I don't think "regular" players will be mucking about in the game's install directory.
If this is a single player game and they player chooses to ruin it for themselves by watching the video early, then so be it. It's their game, they paid for it.
If it's multiplayer, then maybe they'll be able to see something before they get there, but then, they could just ask a player who's already been there. This actually goes for single player too.
So, I'd say don't worry about it. Just make a great game that they'll want to keep playing to see the new videos.