You can add a module to MAVProxy to get this effect.
I wrote this (working!) module so that I could trigger an on-board video camera to start recording from a switch on my transmitter:
https://github.com/peterbarker/MAVProxy/blob/avs/MAVProxy/modules/mavproxy_videoscript.py
You should be able to copy/modify that one to get the effect you’re after.
If we can find a use case which would be common to all installations of APSync, I’d love to have something like this always installed so people will have an obvious base to work from.
Let me know how you go!