A problem that I see is that there are fewer Witnesses than Mormons and they are more spread out through the country. It would be harder to organize a mass resignation like that. It isn't impossible, but it could involve a lot of expensive travel.
A petition might work where people sign it online and provide their congregation. Then it could be printed out and delivered to the headquarters in a way that would hopefully capture some media attention.
It would be a real headache for the headquarters to process the petition and alert all the congregations. If they chose not to, it could be argued that it is still a valid resignation. If they tried to take any action against one of these people later on, it could be viewed as harassing a nonmember and subject them to lawsuits.