A few people have headed down that path, dumping the apparent ROMs that is. Two main issues, the second by far being the bigger one
1) The flash cart may choose to present something different between a DS and a DSi/3ds, and also at different stages of booting (power on vs when the user selects a game/homebrew for instance).
2) Most DS flash carts do not run a ROM as much as a custom chip inside them (FPGA and CPLD classes rather than microcontroller), or something even more fancy. Said things being encrypted and protected nice and good as that is the same thing a would be clone company would need to access to make. This is what allows it to translate calls for the flash cart, save, cheats, text readers, SD card, handle anti piracy protection/checks for the dsi and 3ds stuff and so forth.
Why would you want to make them 3ds flash carts anyway? DS flash carts function somewhat differently to 3ds retail carts, I know that did not bother gateway so much but at the same time it seems custom firmwares have got to an acceptable level so it would be an awful lot of effort for no great gain.
