It could be a plugin. Admins who agree to this statement will install the plugin, and a page will appear listed on site.url/friendica or something with this policy.
Then, this would be listed in installed plugins for any sites listed on the public server list, and, of course, users could find this page on the relevant site. Perhaps with the plugin installed, the new registrants could be directed to this page in their registration e-mail, or while signing up, or something.
Really, most site have you click an "I agree to this ToS" while registering. We could, and perhaps even for our own protection, should have a ToS, where the user is obligated to assume all responsibility for stuff they post, but also informed of our privacy policy, etc. "By registering you assume all responsibility for content posted to your account..blah blah...and here's our promise to you re: privacy..."
Personally, part of the reason I don't open registration is because, indeed, our sites could be used to publish illegal and/or harmful materials. Personally, I couldn't care if dudes want to plan an anarchist revolution (sign me up), but post kiddie pr0n on my site, and I see it, and the FBI or whoever will be at your doorstep in a heartbeat, because I'll call them, myself. Now, I don't monitor what users are posting, and clearly, they have all the same privacy control I do, so I can't even see everything they post, just, I would want it clear: The user is responsible for their own shite. What with RIAA and MIAA trying to hold ISPs responsible for policing privacy, arresting Kim Dotcom for stuff users share, etc., etc., I don't want to incur any responsibility for anything users post. Period. Ever.
But I do want to provide an open but secure and private (sounds contraditory, but it isn't) platform for social networking, communications, etc.