Holy shit Batman! I completely missed your original post on this. This is pretty cool.
Where did you get this? Or, more to the point, where can I get this? Are you in Japan? Can you buy one or two for me there? I'd really like to study this. I think you're really going to want a bunch of games loaded into it to study the menu code as well. You may want varying sizes of games loaded into it. You may want examples of all 4Mb games and all 8Mb games (how big of a game will they let you put in it) and a mix of game sizes.
It's obviously got a memory mapper in the cart. The two major tasks will be to figure out what the memory mapping scheme is (and how it works) and how do you program it.