The SLD PointPlacement Displacement parameters appear to not be implemented when rendering polygons. It would be useful to provide this.
The specific use case is to force polygon labels away from the centre of rendered map images, so that icons rendered client-side on top of the map do not obscure the label, and so the area around the icon is not obscured by the label.
(Polygon labels are rendered in the center of the map image if the map extent is within the polygon feature. This happens when rendering large regional-scale polygons.)