The entire keel is not solid, the walls are. As you say not cored. On mine I can see down into the keels from under the floorboards. I think the bottom of the keel is pretty thick.
It is possible that someone (previous owner maybe) has blown foam down there in yours, I have thought of that since the bilge
pumps appear to have a hard time.
I'm not a pro fiberglass guy, but I think re-gelcoating is a good thing to seal the glass from the water. On the repair job I saw done, they ground down the fiber around the hole, filled it with something like Marine-tex and glassed over it and then added gelcoat
. That hole was about an inch wide from hitting a rock at speed.