I do not know for sure, but it is quite unlikely that you can use a video as background, at least not without changes to the code. (I tried an animated gif, but it does not work Fisume)
A very interesting alternative that I remember from the time when I was testing all different kinds of FPS games was where you start in a "menu-map" (Sauerbraten?). This could be a show-off, demonstrating what the game is capable of

. Pressing ESC opens the menu in front of you, and you aim and "shoot" the options available.