I think ordered hashes are not available in 1.8, so this is not as neat as it could be. But I've added an additional item to the site payload that returns the tags as an ordered set of [name, [post1, post2, postn...]] pairs, ordered by the number of posts.
It would probably be cleaner to make this a method on the existing tags hash, but I'm not sure how.
Yeah, I think so. This could be a custom tag if you wanted it, but not in core. 🔨