Maybe you could establish a blacklist of CP sites, and that could be applied at the entry and exit nodes of the Tor network. This blacklist would have to be public and checked by many that it didn't contain non-CP sites, so in effect it would be a public directory of CP, which is problematic already.
Then, those that run entry and exit nodes could voluntarily apply the blacklist. In this way the Tor community could have its own values, while still being independent of any authority.
But this assumes that CP will remain restricted to certain domains in the .onion system, or the traditional DNS system. Which of course they won't. Maybe there will be one .onion domain per picture. Maybe there will be a Flickr of .onion where it's not so easy to figure out who's doing what. Then you'd have to lean on that service to police its own members' content.
I can imagine various messy and imperfect ways to limit the amount of CP in the world, or at least make it harder to find, but we just don't have good legal models for dealing with true freedom of speech. And our institutions today would rather persist in the fantasy that they can completely control speech, than accept that their role might just be to advise the citizens on how to police themselves.