I'd like to help you, but our boat uses a bronze cap that is keyed to the winch handles.
You mention that you've tried all the usual remedies. In case you haven't tried it, I will suggest that you put a torch on the cap and heat it up. This will often break the bonds of corrosion.
One final suggestion -- If you are desperate to pump the holding tank, you can access it through the inspection port at the bottom of the starboard cockpit locker. (At least, you can on ours, and I do that about once a year to give it a good vacuuming with the pump out hose.)
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Pacific Seacraft Crealock 31 #62
NEVER CALLS CRUISINGDAD BACK....CAN"T TAKE THE ACCENT