I guess I'd still disagree.
While the American Mustang and Burro Association does offer 'registration' papers, they're offered to horses born in the wild, or who's dams were born wild and foaled after capture.
Most people make the mistake of thinking they can recognize a mustang by an ugly head or particular body style. Not so. My farrier came out and trimmed our filly for the first time and was sure she couldn't be mustang until we showed him her freeze brand.
Different HMAs have different characteristics; some are known like the Kigers for their dun factor, although not all Kigers are dun. The Warm Springs HMA has a high percentage of appy coloring. Steens has pintos. Coyote Lakes has thoroghbred and cavalry. Sand Springs will give you ranch horses.
A mustang is a horse who has been born wild. A piece of paper from a 'registry' doesn't change that. Some folks have domestic bred mustangs, but their parents (or grandparents) were born wild. While there are people who are breeding them and tracking their pedigrees, I'd again say that this is really not advisable as long as there are hundreds of horses left languishing in corrals waiting for homes. I'd be very interested in knowing of any other registries that cater to domestic bred horses without proof of being actual mustang.