This report develops a queueing-theoretic solution to predict, for a real-time system, the average-case latency of aperiodic tasks managed by a sporadic server. The report applies this theory to a model problem drawn in the domain of industrial robot control. In this model problem, a controller with hard periodic deadlines is "open" to third-party plug-in extensions. The sporadic server is used to limit the invasiveness of aperiodic tasks on the controller's hard deadlines. The theory developed in this report is used to predict the average-case latency of a plug-in managed by a sporadic server.