I would freaking love it if they released it on DS, but it's not plausible AT ALL.
First off, it would be much, much easier for them to do it on PSP.
It handles PSX-like games well.
The DS is lacking in the cinematics and fancy stuff the PSP has in exchange for a second screen with touch functionality, and handles N64-like games well.
However, if they are insane enough to do it, I mean, they announced Dragon Quest IX(?) would be on it, so...
IX has multiple cartridges, so they could do that if they needed to.
Which they would.