I ordered the valve, but have been thinking the hassle of reaching it may outweigh the extra safety it provides.
I am thinking that the garboard drain seals well enough to keep water out, so it seals well enough to keep oil in.....
The Garboard drain was not connected to anything when I bought it, but it had a hose barb on it and I think at some point in this boat's life, that is what it was used for.