Nav Menu Custom Permission for Editing

Looking for a plugin or hack that allows my content managers to edit only one nav menu under the appearances < menus area. We are using User Roles PRO so can lock everything down to just the appearance < menu area. However, I only want them to edit a designated menu and not have access to other menus for the footer or main nav menu.

I have look far and wide and am at a loss for how to just limit them to editing one menu without the ability to create or edit other custom menus.

Any advice would be much appreciated.