You may have a proper monitoring tool to do this, but if not, you should make an approximate note of how much battery a 1h film consumes, played with MPlayer then Stock. Choose a video that's just inside the stock player's capabilities.
Unless Maemo6 has had a revamp in this area, The stock player uses h/w accelleration, although other video players don't. Result: even though MPlayer etc can disregard the video profile, playback will consume more paower.
- this is all just a warning, when using MPlayer.

As far as making something the default applicatioln (to lauch/use a specific file-type), the mime-type of the video file(s) need to be associated. I think there's some stuff in the Maemo wiki about mime. Look at a desktop shortcut file to see what I mean.