First off: this is purely speculative and I don't really even see what the value of an addon like this would be.
My realm seems pretty dead these days so on a whim I began to just go through a few zones in the /who ui window just to see if people were out there at all. Would it be possible to create and addon that would quickly run a succession of /who queries and count the hits? Some obvious limitations would be the 49 result limit (though on my realm the only zones that ever returns higher than 49 are Org and sometimes Tol Barad during battle) and that it could only return results from one faction at a time. The 49 limit might even be able to be circumvented somehow that I'm not aware of. If not this would be less useful on higher pop realms where more zones would be hitting the result ceiling.
It could give both a cumulative tabulation and a quick zone by zone breakdown. It's interesting to see that on a random Thursday afternoon even all the Cataclysm 80+ zones combined returned fewer than 49 results.
So again I can't see much practical use for something like this, especially if the limitations to accuracy could not be overcome somehow, nonetheless I wonder if it would be possible and if anyone thinks it would be at all interesting to have.