I didn't do any research on it, but you can use air units too in order to keep the observer from getting deselected. In fact, simply keeping an arbiter in an area provides the necessary 1 frame cloak to make any unit invisible (like in rush). Looks like cocoons are sight 4, and nukes are sight range 3 (not certain on the rules for moving nukes)
http://www.staredit.net/starcraft/List_of_units

Combining blind and my method is possible, but it's a nightmare to make sure it's bug free.