or, something that's already done?

If you click the up/down d-pad buttons while a video is playing, it will seek forward/back ~ 1 min.
If you click the right/left d-pad buttons while a video is playing, it will seek forward/back ~ 10 sec.
If you click the center d-pad button, it will toggle pause on/off.

If you click the "menu" button (the center of the three buttons below the d-pad) it will bring up the on-screen menu. the d-pad up/down/left/right buttons will allow navigation through the menus.

NB: this is what works for me, on my n800/Diablo, with mplayer installed from the diablo repositories.

anyway, if you want to change how mplayer (or, for that matter, mencoder) work by default on your system, you can edit/create the .mplayer/config file. if you google for "mplayer config" I'm sure you'll find dozens of posts (as well as a link to the mplayer documentation online) detailing what all of the config options are...