So kill the root of the problem, instead of adding some other rule. I don't count any megathread posts, guitarplinker didn't when Rollin.io's campaign was running, and I'm guessing some other managers do as well. If all managers do this, then things would be fine, without needing to heavily change up how campaigns work, and open up new avenues of abuse.
but how do you get all managers to do this?
The same way you get all managers to require 1 merited post per week. You don't, without theymos making some changes.