There is balsa core in the sump area, it is just thinner balsa. The previous owner of my boat also installed a through-hull in the sump area and failed to seal it properly, and I ended up replacing a 2' x 4' section of the core, from the transom forward.
With that said, it should be a good area for installing your through-hull. After drilling the hole, you want to remove the balsa around perimeter of the hole and fill it back in with thickend epoxy. That will completely isolate the remaining balsa core and prevent water intrusion. The previous owner of my boat simply used bathtub caulk.
-Mike
