The combobox currently doesn't react on the disabled-attribute for options. It should be possible to make single options visible but not selectable with this attribute as in original html-select: http://www.w3schools.com/tags/tryit.asp?filename=tryhtml_option_disabled I found no way to do this yet.
I have complex scenarios where items a hidden because no rights on it and items disabled for many other reasons. Hiding all unusable item and my customer will calling me: "There is a bug, the new element I have just created is not visible..."
+1