Agreed! Definitely useful functionality to have, and to know about 
Incremental controls require something to do the calculation with knowledge of the current state, and that generally needs to be built into the autopilot firmware (like we’ve done with the servo_n_inc/dec joystick button functions in ArduSub).
If there’s an appropriate MAVLink command then the three buttons approach should work with the steps I’ve outlined in this post
I believe that should work, at least until there’s a waypoint that sets a new speed target, or you go out of an autonomous control mode that supports speed regulation.
Note that for the joystick button functions, depending on your QGC version you may need to use the older format for specifying the MAVLink commands.
By the way, I’ve edited your comment to include a link to the MAVLink message in question, and also formatted your file snippet example as a pre-formatted text (/code) block, so the spacing is maintained. The How to Use the Forums post includes a section on what’s available for content formatting