People always like to say these kinds of thing are impossible. Just a few years ago, people said 3DS emulation was impossible, and now Citra is way under development. Same Thing Happened with the WiiU. Same thing happened with Wii. It happens all the time because people are too quick to judge, or are to unknowledgeable on the subject of emulation.
That being said, Here's what I think. I think it's definitely possible, although getting it to playable frame-rates would take a lot of time and emulation accuracy sacrifices. Also, seeing as DS flashcarts exist and work, I think it's very unlikely anyone will start up the project, simply because there's a simpler easier way, at least anytime soon. Also, It'd be way easier and way more efficient to try and load a DS rom into the 3DS's DS mode if somebody ever figures out how to do that, if it's even something technically possible. Until then, best bet is to get a flashcard. Works on 3DS, and if it ever gets blocked by nintendo, you know you can at least still use it on a normal DS or DSi. Unless you are OK with possibly waiting years for another method to come around.