If the picture looks blurry or smeared, then they're using the IPU for rescaling with a linear algorithm.
I guess in theory it would be possible to add support for nearest neighbor for the IPU, they had already added support for bilinear in the yet-to-be-released newer kernel. (if i'm not mistaken)
But right now, the only way to have that pixelated look is to use software rendering. (either by just centering the picture, deboubling or do some ugly pixel scaling, which is rarely done now because it also slow downs the emulator)
For reGBA, you should just activate the border mode. Ohboy, dunno.
EDIT: Oh wait, you want a fullscreen nearest neighbor option ? Sorry but few emulators provide that option, not even ReGBA. (they got rid of it because it ran slower and looked uglier i think)