There's a bunch of parameters called SR0_ and SR3_ that control the rates (in Hz) of various packet types going out on the USB and Telemetry port so you could update those parameters directly through the mission planner's advanced parameter list. Telemetry is on SR0 on the APM1 and SR3 on the APM2 ... I think.
Thanks - I hadn't looked there. I set each of those SR3 parameters to 1 but I'm not seeing anything on the TX pin of the telemetry port.