You should be able to glue plywood reinforcements internally on the balsa through the hole intended for the servo.
If it is impossible to access the servo mount from behind for reinforcement and if the balsa is thick enough. What I have done with good result in the past is to enlarge the holes in the balsa to about 6mm (1/4 inch).
Then I have cut lengths of hardwood dowels and glued then in the enlarged holes, flush with the outside surface. New holes were drilled in the dowels for the servo screws, add a few drops of thin CA in the hole and all this becomes like concrete.