Dramatic Capitol Hill hearings, replete with shocking insider testimony, have brought down presidents and demagogues, put would-be Supreme Court justices on the defensive, exposed war profiteers and revealed deep-seated corruption in the federal bureaucracy for more than a century. But an odd defamation lawsuit filed last month in D.C.’s federal court has the potential to disrupt the Washington ritual of Capitol Hill hearing testimony. And even if it doesn’t succeed, it...