Thanks for the ideas, DgtlSorcrs. I want to be as unobtrusive as possible. Relying on toons being in friends / guild list was something I tried in the early days.

Last night, I had some success with custom channel method that dippy mention. WoW keeps track of who is in a channel, so joining a channel when the addon starts is all I have to do to announce a toons presence. When the toon logs off an event is fired, so I can keep track of toons leaving that way. Polling the /chatwho once in a while lets me catch up with any characters I missed.

And once your toons are in a group, my character online code also checks for same party / raid membership and if it finds that it just ignores the online status. So for the majority out there whose toons are usually always grouped, there will never be any issues of characters being ignored when they are online.