A couple of suggestions...
1) Can the targets be grouped in some logical way that is known to the clients? i.e. Can they subscribe to topics to indicate a group of clients that will always receive that message? Then you can just set up topics where only the expected clients are subscribed.
2) If you don't know that beforehand, but can logically segregate groups, you could have a topic per client that tells that client what topics it should receive.
Both of those assume that you're not worried about unauthorized/malicious subscribers since the client CAN receive everything, it just chooses not to.
If you have to worry about security, then you'll need some sort of auth plugin, and smarts to decide what clients receive what messages somewhere.
One option would still be the single topic per client, where a trivial auth plugin would only allow subscription to that topic or tree. Then the publisher of the messages would have to decide which clients to publish the message to.
Another option would be to put that logic in the plugin, since message sending(at least in mosquitto) is authorized on a per message per client basis, you could publish once, then restrict delivery. That does mean that you're business logic is at least marginally tied to what MQTT broker you're using, since it's wrapped in a plugin and implemented in a compatible language.
What you're not going to be able to do is have some mechanism to direct traffic to one particular client, MQTT just isn't designed for that use case.
-Darren