Well.... you're right on the money pretty much...
there are several ways to do it.... just like about everything else in OFP ;D
but the simplest is to give them a waypoint with 'safe' and 'never fire' where you want them to hang out at first...
and then make the next waypoint 'open fire engage at will' or whatever..
now if you make a trigger.. and synch it to your first waypoint.. the loons will stay there ... on safe mode until the trigger goes off... at which point they will head onto the next waypoint.
you can have the trigger be set off however you want...
but if a waypoint is synched to a trigger.. or another waypoint.. the following waypoint won't be there until the triggers condition is met