Hi Yve, I cannot access the bmp, but on 5.1 there is a "Users Online" in the status bar at the top of the page, so no big deal.
As for the communication without IRC or Jabber, how do you expect this to work? How would you let someone else know that you want to chat with this person. There is the possibility which has been discussed before to use the green report bar for notifications in general which would allow you to communicate with a sepecific user through the user session. But this still has to be developed and I'm not even sure it suits the UI well.
As for HTML/Javascript based chat systems, they suck (personal opinion). But this might be a reason why noone has stood up to the challange and port the chat module to 5.1 or 5.2.