The same way that lists of domains or url patterns get published for tools like adblock plus, it would be easy to publish lists of domains for a boycott plugin.
Yes, this can be done automatically, and I can think of doing it without even depending on a master list: the first time you click on a link, and the browser sees you the page is no good (either explicitly by pressing a thumbs-down reputation button, or implicitly by detecting ads in the page), then the browser remembers to block this site in the future. Then you could share that list with friends or to the world.
I'm glad for your response, as it is the first one that is supportive. My point didn't seem very controversial (ad blocking is not boycott, it is free-riding; proper boycott could be done easily).