I have exactly the same chainplate setup on my boat. I actually was able to pull all (or most anyway) of sealant from around the chainplate going through the deck and even file the sides of the hole slightly wider - then seaed it back so that sealant went back into the deck and down below, as well as above.
The deck around is solid, of course - so no issues with core.
Here is what it looks like cleaned up:
Sealed (you can see where old gunk used to be all over the place)
