Its not a repeater and we are not simply binding to rows it is a hierarchal tree of sitemap nodes. It is more complicated than you may think. You would have to keep track of nodes at the depth that is the top level of the menu and the css classes are assigned deep inside the menu control adapters not in the menu as it is databinding.
I will ponder it but it's not a trivial request as you may think.
Best,
Joe