The client could tell a remote server when it forks a page. I've resisted this because I don't want strangers to write on any site (they will find a way to spam). However, a complementary notice could be ignored, counted or recorded. All are useful options at various states of growth.
The simples recording would be to add a fork action on the page that has been forked. It might be tricky to come up with the reverse address. And it couldn't work when the client's origin is behind a firewall. But this could be tested.
The advantage of the fork/forked unification is that the neighborhood expands in a very generous way.
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There are a number of interesting edge case, associated with some recent discussions (Nov 2013), concerning static sites and using separate data hosting. In both cases there is not an intelligent server component to do anything with such a request.
Any notification probably should be private, so does not automatically expand a neighbourhood but requires a concious decision.
Maybe, the notice should not be on fork but on first content change following the fork.
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If Federated Wiki's origins truly lie in IndieWeb, one day their Webmention 
should be supported. example