The rudder moment is taken about the cg, so must be affected by fore/aft cg movement. So the rudder is less effective as the cg goes aft.
Where is the pivot point for an aircraft on ground? Is it at CoG, or is it at main wheels (which are on ground and therefore not free to slide sideways)?
If main wheels, then the rudder moment arm should be unaffected by CoG.
Obviously, the "loss" of an outboard engine causes asymmetric weight as well - those high-bypass engines are heavy, so CoG would move sidewards away from the lost engine.