I emailed some questions to MB and he was kind enough to respond.
There are ways of gathering metadata from other files. E.g. https://github.com/mbutterick/pollen/issues/57. It's just that Pollen per so provides no pre-cooked high-level mechanisms for doing so. It's expected that you'll learn the basic paradigms of Pollen, and the Racket language itself, in order to build what you need.