No, you can't just assume you have supernatural powers, not even "in theory". If that is allowed, you could just say you are omniscient and thus know exactly what anyone will do, and thus there can be no free will. You have to play by the rules of the world we live in.
There is nothing fundamental about the universe that stops you from knowing the number of jellybeans in a jar. However, there are many things, absolutely fundamental to the universe, that stop you from knowing the exact intial conditions of any system.