I’ve been looking around, but all of the answers seem to be for a different case. Here goes:
We want to have a publicly visible tag that can only be assigned by a handful of members. Everyoene can see the tag on the post, and access the tag URL to view other posts with the tag, but only a couple of staffers will be able to assign it to a post.
Is that workable using exiting restriction options?
Yes, I believe staff-only tags (that is, only staff can add the tag, but it is visible to everyone) are definitely possible and supported.
I think that will work in the near term. In the longer term, when we have a lot more mods/staff, we would still only want select people to be able ot apply that tag. Is that kind of control supported?
No, not yet. Everyone would have to be moderators or up to gain this priv.