Afaik the led on the N900 is controlled by MCE, which settings can only be applyd by restarting MCE itself. This takes some time and can only be done a couple of times every 5 minutes or so. Apart from that, a faulty mce.ini file, where these setting are, can seriously mess up your N900 usability ( read; brick your device)
So in my opinion you'll have to create a new system or program to control the led, and i doubt that is possible...