Apart from memory and graphical enhancement, the SA-1 chip had a raw 10mhz of speed. That is almost 3 times faster than the SNES' own 3.58mhz. The DS has a hard enough time emulating most NORMAL SNES games properly at all. It will likely never be able to play Mario RPG. I can see only two ways it could be made to run on DS-
1- Someone makes a special emulator designed SPECIFICALLY for this single game. Even here, I may very well not know what I'm talking about.
2- Someone manages to reverse engineer the game and port it to the DS. This is not emulation though, but a rewrite of the data. The only game I personally know about that has been almost completely reverse engineered is Sonic 1 from Genesis. Someone named Stealth managed to port the game to GBA using his own created code. This took years of research and the port isn't even complete. Mario RPG is a far more sophisticated game than it.