How can I invoke the after_save callback when using 'counter_cache'?

Question

I have a model that has counter_cache enabled for an association:

class Post
  belongs_to :author, :counter_cache => true
end

class Author
  has_many :posts
end

I am also using a cache fragment for each 'author' and I want to expire that cache whenever @author.posts_count is updated since that value is showing in the UI. The problem is that the internals of counter_cache (increment_counter and decrement_counter) don't appear to invoke the callbacks on Author, so there's no way for me to know when it happens except to expire the cache from within a Post observer (or cache sweeper) which just doesn't seem as clean.

Any ideas?