Sounds like a cool pair of ideas to me

I think the theory for the hatches works fine. There was a National Geographic channel episode on Seal/Narwal hinging in Greenland recently. A couple of the skin boats in that had hatches. They looked like commercial VCP style round hatches. I think I managed to get it on tape. I'll look and let you know.

I like the seasock hatch idea, but what about attaching eye hooks to the inside of the gunwales, keelson and stringsers and stringing a sheet of treated cloth between them. The 'bulkhead' could have an air bladder (out of heatseal fabric) around the edges to seal it to the hull skin.

Sorry about the late response, been on vaction.

Aaron