There's a exploit to find out whether the opposite faction is online, you /ignore them and if it sayd they're ignored then they are online, if it says player not found, they're offline.
Would someone make an addon so that you can /who them *just show offline/online*?
Would someone make an addon so that you can /who them *just show offline/online*?
And that sounds like something Blizzard wouldn't want, I doubt you'll find many willing to code it.