Don't ask me, my answering you automatically made me a thumbs up.
Pledge is designed to work with plastic. You say this is a ball joint, I'm guessing its nylon. If the ball and joint aren't cracked I can't imagine any problems. I might assume this is a ball for moving this on the floor?
If this is gumed by dirt even WD-49 won't hurt plastic. By antique are you claiming this is 100 or more years old. Maybe a picture of the joint would be able to shed some light.
If this is a valuable antique maybe you shouldn't be "tinkering". But if this is a dolly for moving the thing around the floor, its no antique, valuable or not. Pledge will not hurt it, but may gum it as will most oils, WD-40 will clean it.
I can't explain the thumbs down, but I sure wouldn't take it to heart.