I was thinking the same thing. A few smaller items may fit on a single sprue, but anything of a significant size would probably need something larger than 4x6 and/or a multiple sprues.
And not to sound like a broken record, but I've got to wonder how quickly orders would come in for something people may want 1 or 2 of, but not buy en masse.
I've got to admit, it seems like a cool idea, though. I always thought the "Leonardo Plus" games I used to see at conventions looked cool, and it would save a whole lot of modeling/engineering.