this notion of tribe level control with mutes seems to immediately invite a certain degree of centralization.
That is true. However, as explained by @themarkymark a few months ago, in this post, all Hive-Engine tribes and tokens are fully centralized when it really comes down to brass tacks. That's why you really need to focus on the team, in general, and, more importantly, the individual who holds the private keys to the issuing account.
With respect to VYB, all mute actions will be handled by a team (which will be led by @scholaris), each of whom will be granted 'mute' authority by the issuing account (@vyb.vyb), but each of whom will be following a strict and transparent policy. That policy will require notifications to all accountholders receiving mute actions (explaining the rationale behind the action and telling them how to appeal it), issuing warnings for certain first offenses, etc.
Members of the anti-abuse team who do not abide by the policy will be removed from team.