Interesting concept

There used to be folding sinks but I don't think anyone makes them anymore.

You'd need a big, deep boat to even come close to making that head work - the holding tank would have to be beneath the sole.

On both tacks.

As the OP said, there would obviously need to be some re-engineering to turn it into a marine head with a pump-out rather than a "normal" flush. Nonetheless, I can't think of any reason why the overall concept could not be adapted to a marine head. Especially in those boats where you don't have a separate shower stall, I can imagine a fold-away head to be a very nice feature.