I THINK this may work, but Ive never tried it.
Im not making the tail lighter, but making the engines heavier.

Have a read of the "physics constraints" in the manual.
Then when youve got an idea how it works, do this.
1> create physics entities that are invisible AND passable of an un-important shape or skin.
2> use the contraints to "weld" these weights INSIDE the planes engine cowlings,
or where-ever the extra weight is needed.
With the entities being invisible and passable, ALL they do is affect the planes
center-of-gravity by making "heavy points" where the planes mass is denser,
and because they are invisible and passable they'll never cause collisions,
or be seen...

Hope this helps.