In theory: by removing a units actual vision and by placing a dummy unit which is forced to be 500 range away from the unit the way the unit is facing, and having the dummy unit be responsible for the vision, this could work.
I think if I were to fiddle with it I could *probably* get it to work leaklessly in GUI. Unfortunitly my WE completely and totally does not function. (look at sig.)
I tried to make a picture to illustrate my idea. Unfortunitly I suck at drawing (on comp. and IRL)