In the Mirror Universe novel "Dark Mirror" by Diane Duane, Barclay is Captain Picard's personal guard. While on duty (and not knowing that his version of Picard had briefly been replaced by the one from the 'normal' Trek universe), mirror Barclay is murdered by that universe's counterpart of Deanna Troi. In David Mack's short story "For Want of a Nail" (part of the "Shards and Shadows" anthology, which takes place in a different Mirror Universe continuity than "Dark Mirror"), the mirror counterpart of Barclay is a technician working for the Terran Rebellion. He serves as a 'tech-controller' working with his partner, K'Ehleyr, who carries out covert missions for the Rebellion.