Using tagging for this - particularly mod-only tags - feels like bureaucracy for its own sake.
A sufficiently good "canonical" question should naturally migrate to the top of the Votes/FAQ lists in its respective tags. When repeatedly used as a target for duplicate questions, it'll start showing up in the list on the "Close as Duplicate" dialog. If you need a separate list of these, a post linking to them (which you already have) should suffice.
And all of this is either directly accessible to everyone, or automatic based on stuff that's accessible to everyone.
Forcing mods to mediate every single "canon or not canon" discussion just adds resistance to a process that already has plenty of that simply by its nature. They should be able to weigh in like anyone else, putting on that diamond hat only when a dispute requires mediation.