Thanks for the replies and for the compliments on my paint job
Before cutting into the keel, I think I'll try the suggestion from Greenlake / K.C.
If the fiberglass is sound and it's just a matter of the screws having widened the holes they are in, you could use the method K.C. has been advocating:
Wax the screw
Fill the holes with epoxy
Having the screw in the hole will let the epoxy cure and take up the extra room, while the wax will prevent it from bonding to the screw, so you should be able to remove it later.
Will let y'all know how it goes!