Fair enough. Thanks for the input, anyways.
Sure. I just managed to toggle GPIO pin using RELAY commands.
In order to do that you need to set only one parameter:
CAM_RELAY_PIN to either 17 or 18
This pinout will help you to understand what pins are getting toggled. (these are the ones on UART connector).
Steps to reproduce:
mavproxy.py --connect 0.0.0.0:14550 param set RELAY_PIN 17 relay set 0 1 relay set 0 0