how_do_i_do_that's post covered it -- there was an old/beta version of the EZ5 kernel that added support for multiple ROMs on the 3 in 1's NOR. It was eventually dropped as it was causing aggravation with people trying to play pokemon -- pokemon has a crazy large save which means you could run out of save save if you trimmed pokemon and combined it with other large save games. Or if you prefer then pokemon is why we can't have nice things.Hey Schlupi! I'd love to hear how you managed to accomplish this!
With the subsequent bootstrap updates for the EZ5i if you have one of those I doubt you will be able to run the old kernel, if you have an old EZ5i or something in the origin/nye/EZ5 plus line then you might.
Rudolph did also investigate it, especially after the newer 3 in 1+ models gained 512Mbit of NOR, but deemed it more hassle than it was worth. The SDK that is out there should allow you to write some code that could handle pages on the NOR and such, so theoretically you could write your own. Just be aware of the save issue (the 3 in 1 has more than enough save space to handle any commercial ROM of piece of homebrew (which could use more still) but it is quite possible to hit the limit long before you run out of NOR if you start doing multiples -- old NOR carts sometimes got this (see the RAM column in the bottom right http://s13.photobucket.com/user/Slade_au/media/EZClient-Rebel.jpg.html ) and the original models of EZ4 (not sure about the new batch and SDHC off the top of my head) had 8Mbit to handle DS games so they did not have this issue either (though the menu handled the saves so eh).
Short version.
For the aggravation of doing this you are probably better off buying an EZ4.
