The Oryx and Crake idea has been discussed seriously by Nick Bostrom. One thing to keep in mind is that for some metals things will be easier the second time around. The really prominent example is aluminum. It takes a lot of technology and infrastructure to refine aluminum (for most of the 19th century its price rivaled or exceeded that of gold). But, aluminum once it has been purified is really easy to work with. So one would have all sorts of aluminum just left around ready to use. Nuclear war makes that situation slightly worse because a lot of the aluminum will now be in radioactive cities. But overall, you'll still have easily accessible quantities of a light, strong metal that no one in the middle ages had anything like.