What you describe sounds a lot like my plan, and you know the PNW weather. Going with uncoated nylon might be better than the mesh that I was contemplating. It would snag less and protect the inner sacks better. I do want to keep it light, though, so perhaps a very light ripstop.

My wet Ursack was part of what got me thinking this way. I tie my food bag out in the rain at night, so the idea of putting it inside the 50L dry bag was senseless. Packaged food and cooking gear doesn't need to be kept dry.