There's no flashcart as of now that could play 3DS games, but you might want to update to the latest firmware to get eShop, browser and the video capability.
@Xdqwerty, Osu! Tatakae! Ouendan! is the Japanese version of the game, different settings/characters/songs but otherwise identical mechanics. I played that before I knew about Elite Beat Agents lol. Both fantastic games https://en.wikipedia.org/wiki/Osu!_Tatakae!_Ouendan