It's going to take quite a lot of surface rust to burrow far enough into a quarter-inch (or more) of spring steel to cause a stress riser. You'd see much more failure due to rock dents and debris strikes if they were so delicate.
Powder coatings are polycarbonate, and more flexible than steel. Powder-coating lead bullets is becoming one of the latest uses. It stays on the bullet through firing, barrel friction, and often target impact.
As for Mustangs, I'm too brick-headed and closed-minded to comment. Odd that if it's a known issue, Ford hasn't seen a need to address and correct the shortcoming.