Generally looks good. Should some of these be MAV_CMDs? Did you consider making use of MAV_CMD_DO_CONTROL_VIDEO? I believe you could extend it.
I think the next step would be to open a pull request at https://github.com/mavlink/mavlink and additionally tag Don, Gus, and Lorenz. With adding the messages to the common set we could implement it on the PX4 side as well, and then QGroundControl will only need a single implementation.