Maybe I'm overstepping the issue, but my first instinct is to say that if there is a "void" it's best to use a different piece of material. No matter how it's repaired/filled, it will always be evident to the experienced eye that something isn't "right". I suppose it's all in what an individual is willing to accept. Cracks, checks, and splits are givens with certain materials, and their presence, filled or not, is acceptable. On others it's simply not acceptable, and to put a knife out there with filled areas in those types of handles is to advertise to everyone that you're willing to settle for less than the best quality. I'm not trying to belittle anyone who chooses to fill in cracks and voids, but if you do it, and it's done poorly, or on materials where it's not acceptable, your establishing a personal standard that knife buyers and other makers will base the "quality level" of your knives on.
I'm working on handles right now, and have rejected several piece of material, simply because of checks or cracks.