Using empty params of MAV_CMD

I posted this over in dronekit on Stackoverflow but didn’t get an answer so I thought I’d try here…

I’m trying to use a companion computer to control an external device on a copter. I was hoping I could use an empty param of a MAV_CMD as a continent way to pass a value from the Mission that I could then use to set the device for a particular waypoint. In this case I was trying to use param2 of MAV_CMD_NAV_LOITER_TIME. I’m setting the value in QGroundControl, loading the mission to dronekit-sitl, then reading it with Python using the listener @vehicle.on_message(’*’) (so I can see everything). It comes back with a value of 0.0 (empty).

message: MISSION_ITEM {target_system : 255, target_component : 0, seq : 4, frame : 3, command : 19, current : 0, autocontinue : 1, param1 : 30.0, param2 : 0.0, param3 : 1.0, param4 : 0.0, x : 38.7160644531, y : -77.0515441895, z : 25.0}

Is this just not possible?

We only store items that have a meaning to the aircraft due to storage space constraints, that unfortunately means you can’t store unused data, except for a couple of cases where one vehicle type cares about a message and another doesn’t. However that is really rare to find, and not behavior I would bank on being maintained.

Thanks for your response. Makes sense. I’ll look for another way.