actually, you dont need to trim anything BUT the elevator.
In ANY plane, losing a wingtip, just trim the elevator down. The plane ceases to pull voilently to the lost wing side, which allows you to gently fly straight with a little joystick and rudder input.
On P38, navy planes and bombers (aka those with BIG flaps), you can lower 2 notches of flaps to regain a lot of control. This holds especially true in a P38 and Me262.
With elevator trimed down, a nose down pull makes your plane level out, an upwards pull makes it twist towards the lost wing side. Thats why with rudder and flaps out you can keep your altitude and manouver gently and land with no big effort.