create a location over the buidling that you want to cloak, in this case an engineering bay.
Then in the conditions put: always, and in the actions put: set doodad state, and disable the engineering bay at that location.
Because of a wierd bug in sc, the building will cloak. Make sure you have vision to this engineering bay or else it will appear to be unclickable.
Some buildings will crash if disabled.
Also, only disable once, multiple disables can cause crashes.