If you want a 'limited' PointAt, you can try setting the PointAt dial to less than 1. 

Billboard trees will work fine... just move the EndPoint out in front of the tree itself, then set the tree to PointAt your desired camera.

The red cross isn't always visible, so numbers like these in the Joint Editor will do the job...

 

Edit:  Oh, I just realized what you're trying to do.  I'll bet you want to constrain the up-down movement while following the camera left-to-right. 

I remember a long-ago discussion of this.   Seems like somebody, maybe me or maybe not, came up with a partial Python solution.

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