I'm controlling the Basecam by sending Mavlink DO_MOUNT_CONTROL messages from an external processor. I don't see any Mavlink message coming back though except for HEARTBEAT, when that box is checked in the GUI. What other Mavlink messages should I be able to get from the Basecam, and how can I get them? Would really like to get actual IMU angles.
The telemetry data is not send by default. Host controller should send a request: REQUEST_DATA_STREAM or MESSAGE_INTERVAL according to MavLink specification. ArduPilot sends REQUEST_DATA_STREAM as I remember from my tests, and displays gimbal's attitude in the ground control station.
Also host can query a list of runtime parameters and change their values (they are called "Adjustable variables" in the user manual).