I believe there's a sort of automated friendly/hostile setting that's built-in, although it may not be exactly what you're looking for. This setting is normally on by default. In essence, if you try to use an offensive ability (such as firing a weapon) while targeting a friendly, it will automatically switch targets to the hostile that is currently targeted by that friendly. Conversely, if you try to use a healing ability with a hostile targeted who's not currently targeting you but another friendly, it will target that friendly to heal them (this in itself can be a real pain if you're attempting to self-heal and the targeted friendly didn't need the help). I think the setting is called auto-assist, although I can't recall for certain.
Aha! I was just wondering about this since I was unraveling the mystery of targeting option precedence. I'd still like it explicitly added to the Options/Keybinds screen, but this keybind will be my top priority when I login again.