I was told by K&G, and the results matched, that wood with open voids would not have those voids filled during the stabilizing process. The wood in question was redwood burl and buckeye burl. Both came back with lots of open voids, pin holes. As I cut the blocks into scales, there were voids a plenty.
Now, this could totally be contigent on how big the voids are....but some of these were very small....like sewing needle diameter. Other, larger voids, we empty as well.
I've had better luck with wood stabilized by WSSI in this regards...but still similar results.