Hello.
Since opentrack was not working very nicely with Flightgear with 6DoF because it didn't have into account the offset of the current camera view for the axis, i made a custom joystick configuration for it with some nasal code to make it work correctly out of the box!!
the script has to be placed on ~/.fgfs/Input/Joysticks/ on LInux (I dont know the routes on Windows or Mac).
Make sure you are using the libevdev driver on Opentrack for this to work!
All 6DoF axis are working now, no more thinkering with the offsets on every plane!
https://spee.ch/4/opentrack-headpose.xml
Enjoy!!