I'm looking for the same thing. I'm working to make the arduino listen and decode the APM's telemtry, using the mavlinks library. The reading part is easy, but the writing part...
The only command that I had sucess was the "mavlink_msg_request_param_list", where I get a status text saying two informations.
I'm looking for the command long or data stream with the correct method.
mavlink_msg_data_stream_pack(1, 1,&sendMSG,0,1000, 1);
uint16_t len = mavlink_msg_to_send_buffer(buf, &sendMSG);
But I get no answer, and also tried 255 and 0 on the system_id parameter, 190, 1 and 0 on the componet, but no sucess.