Adding weights elegantly
Lead melts very easily, as you probably know. What about melting up enough lead to make the plane balance and then using it to replace a portion of the balsa block under the stab. Shape roughly with a knife, attach with epoxy and blend with sandpaper.
Lead sinkers for fishing work very well for this, and are quite cheap.
Have you tried moving other stuff around to make the plane balance without adding so much weight? 100g. is a lot of lead... There is nothing wrong with putting the battery in the rear, a 600mah pack weighs about 100g. and placed toward the back can go a long way in reducing the amount of "non-functional" ballast needed.