What if it was made in web? That at least would make it cross platform. Would people keep a tab open for it

? I guess not, so it probably wouldn't solve much anything.
Javascript can for sure track for any set limits and make e.g. the window blink to gain attention from user, or make it produce beeps for alert, so largely it should be possible.
Anyway, unless you can ping with javascript, you'd at least lose the lag estimation in the serverlist with this.
It's a bit odd situation in general actually. Is there a good way to notify people about something they want to be notified about, so that it's timely? Something that isn't obtrusive to register for as a user, doesn't need significant system resources to track something, no application installation, no copying files to suspicious locations around your disk drive..
We could of course hook up an automated message in twitter that there are more than 5 players on a server, or mass spam an automated e-mail when that happens to all registrants (not obtrusive at all

), make RWR_Bot keep hyping about it in IRC, have a skype group where to announce it.. and bind RWRWatcher-like functionality running in single machine with single settings from where to trigger an alert/announcement. Instant Messaging would seem like right realm for this stuff, but there's just an insane amount of services and people are spread around.
Right, push notification system, that's the scenario I'm describing of course.