Hurray! I bought a jeweler's screwdriver set today and took the ring off. I soaked the assembly in lighter fluid all day and when I got home I dropped them in boiling water. I grabbed the threaded part with vise-grips padded with a piece of mousepad, and carefully grabbed the other part (which isn't threaded) with the vice grips naked. They came apart.

I figure I'll either use some anti-seize compound, or boat trailer bearing grease on it when I reassemble.