You do realize that this will only be able to go off of the online table used by .threads and not connected directly to an application on the user's computer, I hope. Users may actually be online but if they haven't visited the site in the last 10 minutes you will not see that they are online. The only way a private message will be seen by users is if they visit the forums and see the flashing private message icon. No windows will pop up on their computer such as ICQ if you message them while they are online and not actively browsing the forums. There just isn't a way to get something to work like the ICQ status icon and application works.
