The canonical tag is a bad idea.
Using tagging for this - particularly mod-only tags - feels like bureaucracy for its own sake. The fact that you even have to suggest making this usable only by moderators speaks to the obvious potential for misuse and abuse inherent in the very concept. I strongly encourage you to eradicate this abomination as quickly as possible. I've removed it from all existing questions outside of meta; if it returns, I'll happily blacklist it.
A sufficiently good "canonical" question should naturally migrate to the top of the Votes and 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.
canonicaltag. As an aside, I believe only diamonds should be able to apply certain tags, which is already the case for some tags, and personally feel the
canonicaltag should be on that list. This is because I believe the value of such tags is greatly diminished if not tightly controlled. Some of us have been around long enough to do a fair judgment call but right now I'm not aware of any impediment to a new poster using that tag as they like.