This baby will compute the center pivot of a CornerPin node in Nuke

cornerpin_centroid.py

import nuke

def run():
    cornerpin = nuke.selectedNode()
    xform = nuke.nodes.TransformMasked()
    xform["center"].setAnimated()
    
    f = nuke.frame()
    center = xform["center"]
    xanim = center.animations()[0]
    yanim = center.animations()[1]
    
    # Python ranges are NOT inclusive therefore +1
    for current_frame in range(nuke.root().firstFrame(), nuke.root().lastFrame() + 1):
        xvalues = []
        yvalues = []
        for corner_index in range(1, 4+1):
            xcurve, ycurve = cornerpin["to%d" % corner_index].animations()
            xvalues.append(xcurve.evaluate(current_frame))
            yvalues.append(ycurve.evaluate(current_frame))
    
        mean_x = sum(xvalues) / 4.0
        mean_y = sum(yvalues) / 4.0
        xanim.setKey(current_frame, mean_x)
        yanim.setKey(current_frame, mean_y)
    
    nuke.frame(f)