Ok, let's get real here, a Nintendo DS is part of a 3DS (As some of the processors), so in theory you could make a loader for the homebrew (Like Sudokuhax) but not exactly for the roms (by the time) you might want to ask, why?, simple, the 3DS is pretty similar when it comes to the NDS architecture and hardware, but it has certain gimmicks extracted of the Dsi (Like the Title Menu every-time you turn on the console) and it's in general a more powerful version of the NDS, if you ask how flashcard can run NDS games on the 3DS, it's because the 3ds allows retro compatibility with the NDS, when you run a flashcard on the 3DS, it's as if you're running any NDS game, but everything goes back to the original question: ¿How the 3DS manages to run NDS games?, outside of the fact that the cartridges are incredibly similar, running a NDS game is equivalent to suspending some of the options the 3DS has and lowering his capabilities (such as the processor, the core, etc.) to "emulate" a NDS as itself.
The thing is, you would really need to reverse engineering as fuck a 3DS in order to command and create the option of a "Backup Loader" in which again, you will need a Kernel Exploit, and not any kind of Kernel Exploit like the one we have for 9.2<, i'm talking about a exploit that controls completely the 3DS, from the most basic to the most complex options, while the scene has been doing it excellent to say the least, this kind of thing will only happen when (most probably) the 3DS reaches the end of his lifetime, when everything needed to know about the 3DS is fulfilled, if you still want to play NDS games on your 3DS (Outside of buying the games) take the option of StriderVM, buy a flashcard, they are pretty common nowadays, and new ones work with most NDS and 3DS, try to pick one who has compatibility with 3DS >9.2 (You check that information in the page of the flashcard).