A simple state toggle would be easy to implement for a gadget (including respecting whether you have identified the dot). Together with a "don't/do shoot at unidentified" toggle, that would satisfy your basic request.
So, question is: you have a state toggle, it can have 2 or more states. (I don't recommend more than 3)
State 1: is "shoot everything"
State 2: What units should it shoot?
etc.
What restricted group for a screamer to shoot at would be most useful?
Clearly out: gnat, blastwing, firefly, gull,
Clearly in: shadow, licho, stiletto, vindicator, brawler, blackdawn
Borderline: banshee, phoenix, valkyrie, krow, vamp, avenger