Are there flash carts for other systems like the NES/SNES/Genesis similar to flash carts for the GameBoy Advance that can be written on through a USB connection?
Yeah, I got one tototek MDpro64M some days ago... If you have a printer port Nearly perfect to debug your proggies especialy things that aren't supported by emulators (ea4wayplay for exemple). Good luck.
There are Nintendo official GBA flashcarts, according to Jeff Frohwein (www.devrs.com). He doesn't state much info though, just that his MBV2 tool "can also write official Nintendo flashcarts".