I spray painted a curtain rod in my son's room because I wanted orange and I couldn't find one. It looks fine, but we aren't hard on it- we dont close his curtains often.
No idea in weather the door knob would fit. I would imagine the bolt would be too thick, but maybe get one to try?
Have you seen the knobs IRL? They're pretty, but kind of insubstantial in size. They ball part if just over 1" across. If your rods are standard 2 1/4- 2 1/2" size, the balls would be smaller than the rods themselves. Can you not just buy new finials?