I can see how it would be nearly impossible to replicate fillings as a radio receiver in a lab or on TV, but would write it off as 'never happened'.
Considering the number of people struck by lightning here in Florida (lightning capital of the world), the human body acts as a very good antenna. Fillings vary in composition, there is no standard alloy. All kinds of electrical crap going on in the body, doubt anyone actually understands 100% of it. Your mouth is full of possibilities and variables. Spit (don't want to attempt spelling the proper term...), is an acid, of varying strength, and conducts electricity fairly well, can also be used to produce electricity. Toothe enamel is a insulator, metal fillings are conductors. Unless you drink a lot of hard liquer, there will be bacteria, feeding off all the the stuff hidden in between the teeth.
If a crystal radio can be made from a piece of wire, razor blades, and a chunk of coal, why is it so hard to believe radio waves can be recieved by a mouth full of fillings?