Is it possible, that if you really wanted too, you could build a flashcard that plays ROMs, and just put the ROMs on an SD card, rather then having huge game slots?

I think the point of making the portables out of the systems is to actually use the official hardware, software, game cartridges, etc. Most people would say "If you want a system that doesn't rely on the cartridges, why not just build an emulator handheld or hack a PSP?"

Sounds like the "SuperCard SD" cartridge that I have for my GBA. (The only game I really use it for is the GBA port of robotfindskitten, mostly I just used it for playng mp3s in the car with a tape adapter, and for watching movies and reading text files on the go, back before I had a decent laptop.)