Maybe i have not looked hard enough, but all i see is Line of sight addons for healers. It would be REALLY nice now that arenas are out to make a mod for pesky melee class's to let them know that they are out of line of sight.
What i was thinking was, it would have its own targeting system and you can select players out of your raid/party and then the mod would know who to target. Next would be how to have the mod figure out that the person if out of line of sight. Maybe it will require multiple people with the mod to talk to each other? But, this will help so much in arena combat.
Maybe have the healers have something parsing for LOS (as well as OOR) error messages, which syncs to the targeted player and pops something up on their UI that says they're out of LOS from whichever healer. (or just announces to chat, but that's no fun)