Mark, just add an item with cssClass equal to "k-separator" e.g.:
// more items on top
// more items below
Here is why this works: append (and all methods for adding) calls internally _insert, which in terms calls updateItemClasses which takes care of the item, as long as it has "k-separator" class.
I admit, adding an item with isSeparator or separator option would be a more visible if not natural option, but the cssClass also works just fine.
Here a quick and simple dojo -- http://dojo.telerik.com/@joneff/ElUCo
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