If everybody signs, it's not forcibly, everyone agreed, voluntarily, to all accept that currency.
Good point. Just seems unlikely. Why join a anarchist community that agrees to accept a certain currency when you can create/join one that doesn't require you to accept any particular currency? A community will naturally gravitate to a particular currency regardless.
True, true... he did say he was "just kicking it around". It's just a thought experiment. If the people in a community wanted to start accepting bitcoin for things, I doubt it would really need to be contractual.