I would be shocked to find out that they are anything more than decorative. These are out there at events/e3 and have been shown to the press right? Someone has to have looked into the slot to see if there is even a way to make a connection it it.
Sega has said that they're purely cosmetic. But I'd still maybe be interested in purchasing individual mini carts for a cheaper price ($10-ish?) without having to get the entire Tower of Power.