If I am correct, the weighting of the cv's has nothing to do with the deformer, so I'm thinking you'll have to do it another way. I could be COMPLETELY wrong however since I don't know that much about clusters and deformers. I believe one way you could do it is to project a curve onto your plane, then split the two sections up so the part you do want to be affected by the deformer and the part you don't want to be affected by the deformer are two seperate objects. You can then simply apply the deformer to one of the objects without the use of clusters.

Once again, I'm just taking blind swings here, so don't take my word for any of this. lol Can someone else have a go at this perhaps?