Oh, I forgot to mention ... I recently discovered how to scale my other WoW windows smaller than WoW normally allows on a Mac so they all fit better on the screen.

If you fire up a Terminal session and execute the command:

$ defaults write com.blizzard.worldofwarcraft AppleDisplayScaleFactor 0.5

The next time you start up a WoW client after this, you will be able to shrink its window to 50% the normal smallest size.

You will want to reset the scaling factor after this, with the following command, or WoW will always start up this way:

$ defaults write com.blizzard.worldofwarcraft AppleDisplayScaleFactor 1

There's probably an easier way to have the 2-5th WoW clients do this, but right now I just start up my first client, then execute the scaling command, then start up following clients, and finally reset the scaling factor to 1. Now if only it provided PiP functionality!