Dan, I would simply love to have the wallet required to follow that rule. Unfortunately, I have to look at alternatives that are both safe and affordable. My point in posting was to seek the advice of others to ensure that "may work" means "accomplish my purpose safely". The challenge is finding components that won't corrode, are fuel resistant, and won't leak. I believe the cover I propose will look pretty much like yours except that one would have to remove the sender when using it as a clean out. I don't believe the inside of the tank has seen light since 1986, but I will be sailing to ports in Mexico more than the PO, so 25 year intervals between cleanouts may be optimistic. I'm hoping that regular use of biocides should keep this awful task to a minimum.
As to surveyors, Windmistress and I are pretty much wedded until my Lotto ticket comes in, and that can't happen until I buy one.