You can find the diagram coordinates for each anchor point as shown below, and then render some shape at (cX,cY) by calling Graphics2D.draw(...) from the IconNode.draw override. We could also add a public method to let you run the standard AnchorPattern drawing code from your custom class.

double cX = nodeRect.getX() + nodeRect.getWidth() * x / 100;
double cY = nodeRect.getY() + nodeRect.getHeight() * y / 100;

cX,cY above are the absolute coordinates you can pass to Graphics2D, and x,y are the node-relative coordinates returned by AnchorPoint.getX/Y().

I hope that helps,
Stoyan