The #2 consideration is actually having a community though. And this does require curation beyond merely filtering out kiddie porn. I don't think the concept of a distributed hash table of kiddie porn is enough to deal with this, strong processes for maintaining identities and permissions related to them are necessary.Yes. This is my #1 consideration, which is why I wrote:
The question is how to build and distribute that base blacklist. I'd happily see that sort of thing developed out in the open with the assistance of law enforcement, and anyone else who has content-id on the evil shit out there. Bake it as a client default, a bit like the SSL certificates that come with your browser, and add heavy warnings that you're on your own if you disable it.