Would you like to work on this together?
My area of expertise is industrial controls...not sure how much help I would be creating an advanced browser plug-in such as this.
I know it is a workable idea though. It would be nice as a running backup plan and would serve well whenever the site goes down. It would be very hard to censor and done properly it would be impossible to discern just who uploaded a post. The archives could be held in encrypted form in a directory on the hard disks of anyone that wanted to contribute space.
Moderators could edit and delete posts of course but I can’t imagine a legal doctrine that would require them to delete material from the computers of dozens of anonymous people.