I use a D7100 body with some lightweight lenses (Nikkor 50mm and Nikkor 55-300mm) with the back button as AF-ON, with the trigger in first press in light measurement mode and the trigger in second press for capture. Nothing fancy.
Recently I bought a Sigma 60-600mm and I want to use it by freehand (with both hands on the lens) so I also bought a small external trigger that I attached to the arcaswiss/tripod attach system.
It's working perfectly, I can lift the lens and the camera body without any problem, and I can zoom in and zoom out very easily.
The only problem is that I can't make the light measurement without pressing a button on the camera body because my both hands are on the lens.
So I was wondering if an external trigger support the feature of AF-ON, light measurement and trigger of course. If not, is it possible to Do It Myself by using the GPS port where the actual external trigger is connected ?