The project can be found here. It's feature set is comparable to x7 including no reset saving, game id compatibility with hdmi mods, emulators, and a rtc. Can be built for under $100.
Fxpak (sd2snes before it's name changed) is far and away the best super Nintendo flash cart, and its all open source. emulates the most helper chips and does the fancy msu thing too
I believe fxpak is open source for the snes. I'm not aware of any other open source carts that can load from an SD cards. There are many different ones for the Gameboy that can be flashed using a flasher like the gbxcart (eg. HDR mb30)