During the 2350s and 2360s, he served as a slave overseer on the mining colony Korvat, which was under the command of Gul Zarale. In 2358, he began a relationship with a Terran slave named Keiko Ishikawa. Three years later, she had him arrange a meeting with Zarale, at which she informed him of the plot to kill the Klingon slave overseer Kozak. After the relevant slaves were taken into custody, Keiko suggested to Zarale that he spare their lives as further deaths in Kozak's section would only result in increased inefficiency. He followed her advice.
In reality, Keiko was an operative of Memory Omega who had manipulated Broca into arranging the meeting so that she could find her way into Zarale's bed. Over the course of the next four years, she gradually gained his trust and was able to subtly manipulate him, as she had done with Broca. (DS9 - Mirror Universe short story: "A Terrible Beauty")
By 2378, Broca had become an ambassador. In that year, he signed the Tripartite Armistice with the Terran Protectorate and the Klingon Empire on behalf of the Cardassian Union. (ST - Mirror Universe novel: Rise Like Lions)