This is a great idea Alan, and something I’ve also considered. Surely a sliding rod is a lot of equipment for a simple job? Maybe you could chop off the Yamaha’s moisture-release valve section, and add some plastic piping to take it to the edge of the wooden case. And then add a simple valve, like the Suzuki or Clavietta?
Whatever method you use, there’s always the potential danger of staining/warping the wood when it comes into contact with moisture. I think it would have to be well lacquered…
Looking forward to seeing the results!