This is such a classic give what they want not what they ask for.
What your asking for has nothing to do with trim. But the auto pilot. They plane would not stay in its current state very long with only trim and no autopilot.
You can already set your bank angle by using the rudder while gunning. You now wish the ability to change climb descend angles or at least hold the same.
You can do most of this, by setting auto angle before you jump to the gunner position, the plane will hold the same decent or climb.
Or you can set auto speed, and then change the speed while your in the gunner to climb or descend.
HiTech
I think he is asking to be able to set his aircraft to a certain maneuver (ie a slow roll up and to the left lets say) and then jump into a gun position and the aircraft maintains that input.
Perhaps because gunning and smooth yawing with a twisty handle is pretty difficult?