I did come up with a method of sealing the pithy core that worked pretty well.
After shaping the handle scales, i slathered the pithy parts with a slow cure epoxy. Then put them in a Foodsaver vessel lined with plastic wrap, and used the Foodsaver to apply a vacuum. After about 5 mins much of the epoxy was drawn in so I repeated the process. This worked pretty well and the handle is nice and solid around the pith.