Alt+F4 works when you press i on each window that has the focus

/logout and /quitgame both work I think. Been a long time since I tried either of them, so I don't know if they still work. You could put them into a macro and set on all toons to hit at once like the Hearthstone and set to a key you won't hit by mistake. Not cool if you hit this key by mistake in the heat of battle while in a raid/instance or battleground.