When everyone is absolutely appalled at how horrible your idea is, you know you might be on to something!
I've been powder coating pinball parts and all kinds of other things for a while, and I'm pretty sure I could powder coat a pinball. I'm also pretty sure I could get it as close to round as any other non-coated pinball. Powder coating does a great job of laying down a very consistent thickness. All that said, I'm not confident in how long the final coating would last. I'm thinking it would work best on early solid state games that are inherently less violent. Your captive ball idea would work even better but obviously be limited in how visible it is.
If we suspend disbelief in the concept of being able to powder coat a pinball. What games/colors would be the best to change? I just finished shopping a Globetrotters and I'm thinking a "basketball orange" colored pinball would look pretty cool with the theme.
-Jay