under-actuated chains and setWeightTS

[BrettRD]

I have a 3dof positioning system that only needs to move the end-effector to a point, but doesn't need to care about orientation of the end effector.

From the docs and discussion, setWeightTS seems like the correct thing to use, but I can't get any sensible results out of it. My IK chain converges when I feed it achievable frames including orientation, but fails when I erase the orientation from the target frame.

Are there any working examples of inverse-kinematics on under-actuated (3dof) systems, using setWeightTS, that don't resort to virtual joints?
So far, I've only been able to find complete re-implementations of the IK engine.