Try using the logic in ZH for the Black Lotus, Colonel Burton and Jarmen Kell. They have the blue crosshair where they appear on the radar screen. Make the unit so it doesn't show up on the radar but give the unit the crosshair.
Although if you don't limit the number of units, you could have a lot of blue crosshairs everywhere.
Well I haven't actually tested it yet (it's a bit hard against an AI who's too stupid to tell you it can't see your unit), but for what my intuition's worth it does do what you want. If you know any more please tell me.
