erm, I like the "Dakota/Arizona" suggestions...how about Arizona Cloud, Rizzo for short? Some other great ideas on here though. Best of luck, she looks lovely!
I am sure a name will come up that just won't go away, and it will be the right one. Whatever you call her, Dapple, Silver Pennies etc, etc she won't care, enjoy her, nice looking horse, if she does what you want her to do then she's perfect, and so what if she turns out not to be registered, unless you are were planning on serious showing it won't matter, and if you were you would have got a papered horse.
She looks just lovely, beautifully dappled. I like the idea of naming her after a state too. I think Arizona and Dakota are good.
Also maybe:
Alaska
Georgia
Indiana
Montana