I now know the source of the rusting staples attaching the cabin liner to the inside of the boat -- failed forward hatch bedding. This probably isn't news to Joe but I discovered the problem when I took the hatch off to re-bed it while I was looking for some information to purchase a replacement gasket. BTW I found a manufacturer's sticker on the hatch so I don't think I'll need the AOG brain trust to help me track down a gasket.
Anyway, the bedding was dry, cracked and there were gaps underneath the hatch. But perhaps more importantly I discovered that the liner was attached before the hatch was installed and the edge was wrapped around the hatch opening and left unsealed. So when water was forced underneath the hatch and got past the failed bedding it was absorbed by the liner which essentially is a thin sponge. The so called "bedding" was in such poor shape that I think it was cheap house caulk. Other than a thin layer which stuck to the hatch surround, most of it flaked off in large chunks.
Anyway, cleaned everything up and re-bedded with butyl tape. When I install the new gasket I'll test everything with a hose, but at least for now things are better than they were.