They are very beautiful and unique horses. I found an interesting site while searching it but I don't know if the stally is actually proven or if they are just saying that. They have some very interesting colored brindles though. I want the sorrel one.
Yep. I first heard of the color when I read Riding Lessons by Sarah Gruen, and the main character had a brindle Hanoverian show jumper. Here's an article on chimerism, which I think causes the brindle pattern.