Sorry for joining the discussion a little late,
let me just drop a few thoughts, maybe they can be useful in finding a good solution.
We have deliberatly avoided any kind of audio signals besides the music playback - if any kind of notification came with audio and you were just recording the browsers audio output (which is a totally valid way if you are too lazy to download and import), then such a sound would ruin your recording.
Now you will say: "Yes, but maybe one can disable them" - the thing about that is, people tend to not-find offered settings, and one would have to come up with a default setting. I'd vote for "no audio" as default, but that would lead to no one but maybe those who have read this thread using the feature.
Second, I do see a privacy issue here, too.
Being listed among the "online users" is optional for a reason, if you do not want to be bombed with messages, just go and hide your online status. It is a necessary feature, put yourself in the shoes of our lady singers, you do not want to get all the spontaneous requests.
What we do have: In your band-spaces, there is a seperate listing who of the band participants are online, maybe one could meet your need starting off from there, that would ensure that people "opt in" to being "exposed" this way.
A second approach would be to simply sort the online users panel, so people you are frequently messaging with appear at the top of the list. This will be a little difficult to achieve, but it can be done (alternatively, one might have the option to "pin to top" some of the users in the online users panel.
The downside of this approach is that the online users are not being displayed on mobile devices.
Bottom line, I'm aware wikiloops does not really work ideally for instant-messaging purposes as skype (or other services) do, and if there is a need to improve on that, I can see what can be done in mid-term.