I've seen such animals, with one exception. The carapace apparently doesn't always have to be so dark. Here's how I understand the "true" major as a subspecies. It's a factor of both structure and behavior. They are closer to a water turtle than any of the other box turtles both in appearance and habit. They have relatively flat, hourglass shaped shells with dramatically flared margins...mainly the rear and above the head. They are large, with long legs and neck. Lastly, and fascinating to me, some will swim underwater, eat underwater, and even hibernate underwater! And like you said, white on their heads...sometimes even grayish or blueish.
Back to the question at hand, "are these present in any collections?" I think that this species is rare in the pet trade because of the legal protection that it receives in its range. For most other species of box turtle, there is at least one state with large populations that allows trade. Florida allows the possession of only two Terrapene carolina, and I don't think they differentiate between subspecies...I may be wrong. Most people want bauri out of Fl, not major. Then, Louisiana has just fairly recently passed some legislation although I don't really know the specifics. I don't think that people have made the effort to establish true major captive populations like they have with the other subspecies. I personally know of only one guy in Texas. I've asked him to send me pictures, but he's more focused on rattlesnakes. He said that he just keeps them because he enjoys them.
Here's a site that I like mainly for the side by side comparison photos. I don't agree with some of what's written.
http://www.pogospals.com/pogospals/Box_Turtle_Photos.html
I'm sorry that I'm not able to help you out. I'm in the same boat.