Maybe this control exists and I'm ignorant of it...
I do a lot of driving in my panzer from the pintle gun (using the rudder controls). I would find it VERY convenient if I could also rotate the turret from this position.
I find myself having to pop in and out of the turret so I can ensure that I rotate it to the proper angle for a shot. Usually this isn't an issue because I can just turn the tank towards the target, but if I'm somehow imobilized, or I need to track an airplane, this can become a real pain.
If this doesn't exist, I would like to suggest mapping the aircraft trim controls on the keyboard (J, L, I, K) to this function. That way we can do a rough pointing of the tank cannon from the tank commander's position prior to aiming our shot.