As it is, the large majority of players with their stock firmwares rely on computer-side applications that convert everything down to the proper resolution and codecs for the player so it's minimally tasking on the processor and doesn't have to waste resources on converting and resizing on the fly.
I certainly wouldn't expect rockbox to accomplish much different. This is the prime reason why the documentation for mpegplayer calls for re-encoding your videos for the native resolution of the player.