idlerpg?

[ZiNgA BuRgA]

Unfortunately, a bulletinboard is not IRC, and idling is nigh impossible to do with a forum, since there is no persistent connection (the connection is closed once the page is downloaded).
FYI, the "online/offline" status is really just a guess, and based on past activity - it's impossible to tell whether someone is really looking at a browser page or not, and also, whether they have it open or not.

There may be a minor possibility with AJAX, but that in itself has a myriad of issues.

It sounds like an interesting idea, but unfortunately, technically unfeasible for a website.