what would be the point?
Moonshell. if your just wanting to watch videos.

 

If you're talking about modding a retail ROM... unlike older ROMs (NES, SNES, etc.) that lumped all the data into one chunk, NDS ROMs actually use a filesystem of sorts, and there's tools out there that can extract the ROM into all the various parts, which include the ARM9 and ARM7 binaries and also all the resource files for the game. This means all the text in the games and stuff like that can be edited without modifying the program itself (assuming you're just doing simple resource swapping).

 

Ahh it becomes clearer- the phrasing was enough to attract my attention but not convey quite what you were going for

Graphics wise- if you have seen subtitles on a video within a game that has been translated then chances are the subs you see are floating on top of the video itself (there are no encoders, public or otherwise, available for any of the DS codecs/video standards that I am aware of). Bonus for you is MKDS is largely 3d so you might have less of a hard time fiddling with priorities (3d is rendered outside the normal VRAM and kicked to a BG layer for the most part).

Your main problems as I see it will be
1) Trying to get your code in there. Personally I would look at hooking the 2d display routine and adding it there.
2) Trying to find the space in VRAM. The DS graphics system can do a lot but it is far from effectively unlimited* and certainly no stranger to having to be optimised to get things to work.
*It is for the GBA but http://pineight.com/gba/managing-sprite-vram.txt is worth a read and http://gbatemp.net/topic/295053-resize-sprites/ has a tiny bit (mainly a lack of VRAM issue). If you nuke the map and such it will probably be a bit easier.

2a) would be that there might not be enough space in the vblank timeframe to get things done but cross that bridge when you come to it.

As for controls what you probably want to be looking at is the Arkanoid paddle for MKDS hack http://gbatemp.net/topic/75311-control-mario-kart-ds-with-arkanoid-paddle/ and http://crackerscrap.com/docs/sfchacktut.html but in reverse.
Personally I would ignore graphics for now and do the controls as that would be the harder thing and also a good proof of concept to see if it is worth pushing forward.