- Freenet would work nicely, but it's too difficult to use. Requiring it would prevent most people from joining the discussion. (I don't even run it any more, since it takes up too many resources.)
Freenet is the best choice there is, since it's its main purpose. If you designate the meeting place as a freetalk channel, it really isn't a pain to set up (AFAIK it comes with the default installation now). Freesites are extremely resilient as well...
On the other hand, you need to set it up. If that's too much of a demand, big enough social networking services seem like an option. But what if the attacker has means to intimidate them?