You can get carts with parrallell port connections from Tototek (http://www.tototek.com/), they work with the 32x as well. I'm not overly keen on them myself but it's about the closest you'll get to what you're looking for. If you want to go the retro route you're probably better off trying to hunt down an actual backup unit, although Megadrive ones are nearly impossible to find these days.
Like FAST says, emulation for the MD is great and is probably an easier and cheaper route until someone designs a proper flashcart with removable memory.