While the wikidata summaries are not implemented I ask that we remove these from the specification, or at least add to a draft section. I'd like to talk about them some more. I'm not convinced they will be needed, right now.
Topic on Talk:Page Previews/API Specification
I don't mind putting the section in draft. I would mind deleting it as adding Wikidata support is an FY2018-19 Q1 goal for Readers Infra (see Wikimedia Audiences/2018-19 Q1 Goals#Readers).
Draft section sounds good to me!