I've done the following procedure on my boat for any for any through deck fittings (including the chainplates) that have been leaking.
1.If the core is wet, scrape out as much of the wet balsa as possible.
2.Dry out the core. You can use a heat gun on a low setting but be careful because it's possible to burn fiberglass.
3.Put a piece of tape on the underside so the epoxy can't run out of the hole.
4.Mix together the West System epoxy with the hardener for 2 minutes. Add enough 404 filler to make the epoxy thick - somewhere between the consistency of ketchup or peanut butter depending on whether you plan to use a syringe to pump the epoxy mix into the hole.
4. Inject it into the hole. If the hole is deep, don't try to fill it all at once. Sometimes you can get a big bubble in the curing epoxy which is undesirable. Try several layers over a period of time.
5. When the epoxy is cured you can drill or cut a new hole for the fasteners, etc. Use donuts of Sikaflex or 3M 4200 around the holes to bed hardware or chainplates.
This will seal of the balsa core from future leakage but I think it reads oddly on the moisture meter.