Attrition is also in Rhye's mod, specifically for non-Russian units in Russian borders.
I would definitely like to mod in attrition for units in enemy borders or hostile terrain (and to a lesser extent neutral terrain), but I don't have the knowledge base just yet, and am mainly doing extensive xml editing on the modding front. I also wouldn't mind latitude-based attrition for sea units in ocean squares; it would make that trip to the new world a bit tougher on your caravels (and kill off any pre-modern naval units hanging around the poles for too long).