I've seen, and used, a similar technique to what Dan was talking about. I've used stick on weights for initial balancing and, once the balance point was determined, I took off the stick on weights, weighed them in total, drilled out a piece of balsa and poured in lead fishing weights and epoxy until the combined weight was the same as the stick on weights. I then glued this in, giving a balance point just aft of the determined location and then rebalanced using internally installed stick on weights. A test flight or two to verify the set up and your done. I've found that, when using weight that is hard mounted in this manner, it's easier to set up the plane slightly tail heavy so that you can trim it to what you want with a minimum of additional weight