I don't see why it should not be feasible via a plugin, it's not like there is anything special or revolutionary behind it.
Though, of course, it's a bit of work, you would probably need to sacrifice something like 3-5 days on that. Basically you would have to read the JavaScript client code and implement the network packets sent in terms of C++ and EditorEvents.
Since you can (or rather, have to) ignore styles like bold or italic, all you need to take care of is insert and delete mesages plus join/leave messages, which should not be too harsh overall. Plus, you would need to add a small window to the management pane where one can see the per-user colour code.
The biggest problem (apart from time and interest) is the fact that the source is only available via Mercurial (which, if you're like me, you might not have).