Flaser, the command briefing is part of the interface.

And note that I said
any movie. This includes ffmpeg, ffvp8, and h264.
First of all, you have size restrictions. Theora frames need to have dimensions that are multiples of 16, so right off the bat you won't be getting an exact match to the cbani drawing area. Second of all, you're going to run into trouble painting the movie on top of the interface because it's designed to play on its own, either in a window or full-screen, and you'd have to develop some sort of matte system. Third of all, the movie playing code operates in its own code module with its own drawing routines, which are not going to play at all nicely with the main game code and its drawing routines. Fourth of all, decoding of any movie format is based on the encoded timer, not the timer used by the game loop, so you'd have to keep the movie code in its own separate thread and deal with the huge synchronization can of worms that that opens.
I can tell you right now that your efforts are going to be much better spent developing the next-generation ANI container format. I can't stop someone from going down the movie playing rabbit hole if he really wants to, but I fully expect him to burn himself out proving that it doesn't work, and then we'll have wasted energy, time, and effort that could have been spent more productively elsewhere.