I do like your idea, and think it's pretty interesting, but I guess I'm pretty old school in that I like the who's online list. Whether or not someone knows any of the names of the people when they scroll down to see who's online; if there are a bunch of members that are online, it may help to make newer members feel that it may be an active community- thus maybe wanting them to become more engaged themselves.
In my opinion, when I log onto a site where the list of online members is disabled, it just feels sort of cold and almost as if I'm on the forum all by myself.
Got to agree with both messages, lol
That looks like a rather subjective thing: what I usually do to guess if a board is active or not is a quick look at the dates of the topics, if they all are from ages ago it means the board is dead, otherwise is active.
The idea would actually make who's online more prominent. I would put it on on every page since it's only loaded when you click on it via JS.