Most anti-billboard laws are in place to limit eyesores, improve traffic safety, and keep developments in scale with their surroundings. If a community has been prevented from eliminating such eyesores due lobbying efforts by well-funded commercial interests and the mistaken notion that corporations and commercial entities are "persons" with free-speech rights, as is the case in Portland, than I see no reason to object to a neighborhood members modifying a particularly offensive billboard in an attempt to make the community undesirable for a capitalist invasion.

Of course, I've always been a fan of allowing more information into public discussions, even if that requires acceptance of unconventional means (like grafitti) to give otherwise-underrepresented elements a voice.