the usb led light doesn't set/get information on the usb bus, it only take the necessary power to turn on.... so there is no direct way to control it...
the only way is to make a circuit to make a serial/parallel port as a switch for the usb port though a usb switch build for that reason.... but in that way you could make the circuit and script that some other user made with the same your idea: flasher script (http://shoutbox.menthix.net/showthread.php?tid=61488)