Thanks, that worked! I have not read about query actors before. Problem is, if the Broodlord gets in range of another unselected one, its broodlings get highlighted too.
So I think I'll go another way:
1. I'll add a conjoined behavior to broodlord and broodlings,
2. make the broodlings selectable,
3. hide the selection circles of the broodlings with a z-offset of 100 and
4. add the messages from my first post to all units (StatusOn.*.Selected; HaloStart and StatusOff.*.Selected; HaloStop)
Ugly side effect is, that now you see the move order commands of the broodlings.