Hi,
This action is pending since a long time, and I got around to finally do a version of it.
Basically, some time you want to look at a target, but keep the object rotation axis locked, regardless of it's orientation ( a turret on its tank).
There is an additional check box called "Constraint Locally" which in effect makes the "Keep Vertical" option local to the gameobject, as opposed to taking the world vertical axis instead.
You can find a fully working example here:
http://hutonggames.com/playmakerforum/index.php?topic=4908.msg23062#msg23062Bye,
Jean