The only thing I'm not sure of is that most cops are generally seen as enforcing just and/or democratically-approved laws. Maybe I'm overly optimistic about that, but "canon police" loses a bit of the original impact by leaving out the nuance of injustice (and cruelty, and horror, etc).

It's better as a self-referential term, especially when it comes to issues of canon. There is much more of a solid line between what's canon and what isn't than, say, policing nebulous ideas of "community (proper) behaviour".

It definitely flows better than "canon pedant", that's for sure!