Movie playback is already at it's true resolution isn't it? There's code in the gp2x mplayer that sets the correct resolution at any rate, and I always assumed that gp2xmenu sets the correct parameters when it calls mplayer. Maybe you're not launching it through the normal route? If you launch it in any other way then you should make sure you're running 'mplayer tv PAL' or 'mplayer tv NTSC' to get it running at the correct resolution, as it will default to 'mplayer lcd' otherwise. There's also a 'mplayer monitor' option that sets it to 800x600 - maybe GPH had vga planned at one time.
Anyway, back to the original question - many devs use SDL which automatically supports tv out, so I'm not sure what makes you think devs aren't supporting it. Is there still anything other than early apps (written before the tv cable became available and never updated) and the odd emulator that messes with things it shouldn't that doesn't support tv out?