The catch-all is that if you break a state law, you've broken tournament rules by default since every tournament worth fishing has a rule that requires you to follow all state and local laws.
So, catching and keeping a 12-inch fish on the Missouri side of Bull Shoals would be no different than setting out a 20-pound basket of bass, retrieving them and weighing them in. Either way, you've broken tournament rules.
Yes, cheaters are going to cheat and you're going to have some try to get by with it in tournaments. We can only hope that self-policing and polygraphs will deter most of it.