Hm,
BanzaiAlert seems to be able to warn you if someone targets you, even if you haven't targetted him itself.
I.e. even in PvP it will show you the alert if some enemy player has selected you.
I don't know if it depends on other players having the same library, but I suppose since it works even in AV, it's something other and it seems not to depend on the combat log (as in, he just targets you and doesn't perform any action that is showing up in the combat log).