This will be a feature of StuffPlug 3. I know that is not a good answer to the question, but you might be interested in it.
Also keep in mind that the method described above will influence the response of the keyboard. It doesn't really flash the lights, but instead it turns num-lock on & off, and that's causing the LED's to flash.
However it can be quite usefull. It's one of my favorite "small but very usefull" SP features