That is a great suggestion. You’re right. PressBooks doesn’t have that kind of capability yet.
At BCcampus, we are using the WordPress plugin CC Configurator to add CC license information to the book, which also adds the embedded metadata to the book. But that only covers the actual book that is created. We are still manually adding in CC license information to the books when we use existing content, primarily images where we add the CC information as the image caption. We have also extended the CC Configurator plugin to add derivative information to the book when we have made a derivative copy of an entire work.
So, for example, we are creating a derivative version of the OpenStax Intro to Sociology textbook. We have imported the book into PressBooks and are localizing it – going through the book and adding in Canadian specific content. At the footer of each page, we have altered the CC Configurator to add in the derivative information stating that our version of the book is a derivative of the OpenStax book, and the terms of the original license and the new, derivative license.