Yes, I'm discovering the joys of washboards. I have 3 and they tend to stick a bit.

I have always thought that a hatch on an ocean going vessel should be watertight. Preferably even if the boat rolls over.  (would the RNLI / RN have anything less)

Washboards and the like are for day-sailors! ?

I have just had a look at your photograph and it is not simple is it, partly because you have such a deep companionway and also because the sides are not parallel. I think I would start by glueing in the lower white board and then maybe creating an extension of the cockpit seats across, maybe a foot or so wide, as a small bridge deck. This could make for some useful extra cockpit stowage underneath, provide extra seating, and because the lower board will protrude slightly above this bridge deck by several inches there will be added security from any waves that might one day find their way into the cockpit. This would then reduce the area of the open companionway. For simplicity you could then retain the current washboards, or a modified version, and have made up a zip up/roll down/whatever clear flexible closure for use when you are onboard and don't need the full security of the solid washboards. Or once you have created the bridge deck as described above you could put a narrow infill into either side of the remaining companionway opening to make the sides parallel and then have a 3 part washboard type arrangement that folded down as per your idea and sat on, or slightly into the bridgedeck as you described. I am just not sure how you would arrange the hinging for this. 

I agree with your sentiments about washboards, terrible things and where do you put them when they are out? I have been toying with the idea of some swing out doors which are mostly windows for Footprints to replace the current washboards, but with everything else I have been doing I just have not got there yet. I do recall though that David Tyler had a good arrangement on Tystie where I think the single 'wasboard' slid down, and then pulled up to close the companionway opening. I don't think there was a bridge deck as such but a very high sill which was I guess half the height of the companionway opening.

I think that what ever you do, and me eventually, it has got to look good, be functional, be secure, and most importantly keep the water out.