Looks even trickier. I think you need to find a way to get the seal spinning. I still have bench top machine tools so I would make an arbor shaped like the end of the piston to hold the seal in the lathe. In a larger lathe, you could chuck up on the piston body itself. Could you find a seal adapter that you might be able to mount in a drill press? Perhaps the type with a center hole used to convert leather seals to plastic.