I wondered about that, too. I assume that anything said in Congress would be protected by some kind of absolute privilege*. So he could expose any secret goings-on that he could find out about.
*To be honest, I don't know the US constitution, and I'm extrapolating from the British model. Statements made on the floor of either house of Parliament are privileged. Presumably the Official Secrets Act doesn't trump this privilege - how could it, when the Act is Parliament's creation.