Hi @Stihus and welcome to the forum!
We have tried something similar to what you are planning to do. We have attached a crawler to the bottom of our BlueRov2 Heavy and are controlling it with the LT and RT buttons on our XBox controller.
The problem is that LT and RT are not actually buttons, they’re axes, and QGC does not recognise them. (I believe this is because some controllers don’t have LT/RT axes and QGC wants to be independent of controller type.) This also means that you would need to change quite a lot of code in QGC to be able to use them and pass their values on to Ardusub/Pixhawk.
Our solution was to separate the crawler control from QGC entirely and instead use ROS and the pins on the Raspberry Pi. So now we are running a little ROS node on the Pi (see here for Installing ROS on BlueROV companion) and another ROS node on our laptop. The ROS node on the laptop takes the joystick controller input using joy and then sends a custom message with the direction and duty cycle to the ROS node on the Pi. The ROS node on the Pi then sets the PWM signals on the pins of the Pi.
The ROS code runs at the same time as QGC so we can control the BlueROV normally and make use of LT and RT to control the crawler.
I hope this helps you to get started. Let me know if you have any more questions.
(Note that this project is still under development so I will not be able to share any of my code.)