"on-line" is a very difficult metric in a browser game, because there's no guarantee that the person is actually interacting with the game, but "last activity" would be feasible.
There's also a HUGE privacy issue, and you'd have to provide a flag that allowed folks to conceal their status. If somebody is just ducking in for a few minutes, to clear their accumulated Knowledge Points, they may not have enough time to be courteous.
I'm a LOT more comfortable with using the Fellowship Chat as a sufficient indicator that someone is available for casual interaction.