I don't think your idea of drilling into the backing plate will work. I haven't been inside that part of a 19 but I think you'll find the tang goes into the hull and crosses over to the other side. It does not go down the inside of the hull.

You can check that by taking a razor knife and carefully scrape around the bottom edge of that opening. If the plate goes down you'll be able to feel it with the blade. If it slides in without hitting then you can assume it crosses over.

Some times you can cut a hole to gain access but it this case if the tang crosses over it's most likely bolted to a glass plate that's bonded or glassed between the hulls, meaning the deck will have to come off to gain access to those bolts.