The 'cheap' way to do it would be to have the Bears orbit until the player shows up. The two drawbacks to this are that they would never get too close to the CVBG and that if the player caught them on the way in they wouldn't turn back.
The expensive way to do it is to use the 'alt-path' for AI aircraft trick. The problem with this is that you can only do it for one plane, so the player would always know where the threat was coming from.
Clearly there is a third, economical, way to do it. The user flag appears to do no more than set up the mission-success condition. Perhaps an unarmed aircraft on a fighter sweep that encounters an enemy quits its action and goes to the next waypoint.