I think this is a great idea.
I think it would be interesting to have a billboard that says something like, "Did you know that Jehovah's Witnesses have a do not call list?" and then list a website with a simple url like jwfacts.com.
I think this is one issue that affects nearly everyone because most find it irritating to be woken up or bothered on Saturday mornings. Unfortunately, most people don't care about the internal beliefs or practices of Jehovah's Witnesses, but they might show some interest if they can just learn how to get them to stop knocking on their doors.
I don't think that message is hateful or libelous in any way. Some will go to the website and learn more. They might even give reasons for asking to be put on the do not call list. This can really have a powerful effect on the visiting Witnesses. Having to keep track of do not calls can also make the task of witnessing less efficient and more discouraging, in my experience.
If you wanted to be more aggressive, you could say something like, "Is there a pedophile knocking on your door? Learn what you can do about it." and then provide an appropriate url.