The basic idea isn't tough: there are events in Lua that trigger when a unit enters vision. What is tough is deciphering when to send a message: when the first bomber is in vision? When the fifth is? And what if you lose radar, and they get back in vision a few seconds later? Do we resend the same message? And what happens when there's two squads of bombers at the other ends of the map? Do we send two messages?
Technically it is quite easy: the event is there. But sending the right message / ping / sound queue with the correct interpretation of the situation is what sounds difficult to do right.
This is the event:
The event happens in the sim, and therefore would require a sync operation for the UI to be aware of it.