You can find updated version on the PM page. Now there is a DiagramView3D.GetProjectionRect(DiagramNode) method you can use to find out the client area coordinates of the node projection. You could use it for hit-testing from the view3d' mouse events, or to custom-draw from the Paint event handler:

private void diagramView3D_Paint(object sender, PaintEventArgs e)
{
	if (diagram.Nodes.Count > 0)
	{
		DiagramNode node = diagram.Nodes[0];
		Rectangle projRect = diagramView3D.GetProjectionRect(node);
		if (!projRect.IsEmpty)
		{
			e.Graphics.ResetTransform();
			e.Graphics.PageUnit = GraphicsUnit.Pixel;
			e.Graphics.DrawRectangle(Pens.Red, projRect);
		}
	}
}
 

You don't need this method if custom-drawing from a DrawNode event handler; in that case the Graphics object has such transforms applied to it that e.Node.Bounds corresponds to the projection rect inside the 3D view.

I hope that helps,
Stoyan