a little to late but here is an all SOPs and CHOPs based approach Rob and I worked out:

after converting all points via a SOP to CHOP you calculate the distance off all points to the points that get initially transformed by taking the absolute x and z position of each point and then combining x and z via choosing the maximum value. The distance from the originally transformed point is then inverted and multiplied by the amount of the original transform. That was the solution to getting surrounding points to the same height.