Something like this might work:

adult-sized banana seat

It comes with its own (oddly-sized) seatpost. Maybe you could shim it to fit. I can't see the mount in the pic but it looks like it might be a standard two-piece seatpost like you see on cheap/old bikes. It probably would be far cheaper for them to have adapted the design to one of those (buy from taiwan for 3 cents) instead of fabbing their own custom piece.

Worth a call, maybe.