Here is an early beta version, which actually works in the xy plane, for 3+ planar poly objects.
Further testing needed.
Test 2D polys in 3D space, Poly triangles of portion of an icosahedron, is working.

Copy CentroidNode.js to nodeeditor\nodes\extensions folder.

For Testing, open Ico5Poly.3dm, of color Orange.
Also Load PolyCentroidNodeTest3.nod into nodeeditor, or some other test .nod, and Run it.

The extra point at the origin is to be investigated. EDIT, the pointarray centroidPt (pink node dot and pink wiring), is defective. (Should not be curve mode either.)

- Brian

Earlier a strange effect occurred. An icosahedron was loaded from Obj Library. 15 edges of 5 side faces were joined to form 5 polygons. Did a 2X scale of, and moved, the "Ico5Poly"
Icosahedron was deleted.
Running the CentroidNode found the centers of the Ico5Poly, But strangely also caused the orange icosahedron to re-appear.

Edit: See post 1456 for beta3.