Michael is absolutely right that we do not offer an encompassing theory of copyright's domain in The Piracy Paradox. In part we felt we had taken on enough by tackling the links between fashion and IP. But at the same time we wanted to highlight what we saw as a deeper question at the core of IP theory generally. With limited space (and intellect), we merely laid out the question and offered some food for thought.
It is nonetheless striking to me that the project of delimiting what is in--and what is out--of copyright has received so little attention. Certainly others we cite in the paper, such as Jessica Litman, have pointed to some anomalous areas such as cuisine. But positive theories of copyright's domain are scarce. In part I see this as a species of a broader genus. Legal scholarship is often consumed, perhaps understandably, with what the law is. Fixing broken doctrines and explaining how they fit together are common projects in the legal academy. But less common is the project of delimiting when law is used or applied and when it is not. (In the other field I write in, international law, scholars have written reams of articles on treaties, but very little on informal, tacit, or non-binding agreements.)
Certainly work of this sort exists; Lisa Bernstein's fascinating paper on the diamond industry comes to mind, as does the story of Shasta Country ranchers told by Robert Ellickson. More is needed. Chris argued that we need to do this inductively and have a granular understanding of different industries before we go forward. I think a deductive approach is also possible. Either way, I'd certainly like to offer more thoughts on this in the years to come, and I hope others do as well.