It's like any other burl. It's so busy you should be able to fill the voids with most anything and make it work. I would be inclined to put some finish on some of it to see what the colors would be and mix some bondo in different colors that match the finish color and use that to fill with. You can use a universal tinting color to tint bondo, just make it a little less red to allow for the red in the hardener when you mix that in. You could also just use different colors of wood putty but if the voids are very big may take several applications to fill.