COMMAND_CANCEL support

I have 2 devices which communicate by passing messages through the Pixhawk ArduCopter.

It appears ArduPilot is not capable of passing the COMMAND_CANCEL message through to my 2 devices. (atleast on 4.2.2 which I currently use). I don’t see COMMAND_CANCEL in the common.xml arudpilot uses(or anywhere in the project), therefore ardupilot can’t even see the target_comp I am trying to speak with.

Are there plans to support this message?

Is there an alternative to this command that I can use until support is given?