That sounds annoying, but your characters will most likely be pretty close - are you losing much if you just center the cast on yourself?

Edit: I mean, since you're boxing your slaves will be close together most of the time, so a mouseclick (mapped to a key) in the middle of the screen would hit the group 90% of the time. You're missing out on hitting the tank, but I guess you would as well, even if you targettted manually?