It is not possible to make a macro to target someone based on HP.
Along with binding strafe to a&d and leaving nameplates on I do tab target alot, and for that I use Smart Targetting.
(If your too lazy to click the link, when you enter a BG or Arena or Guru it auto sets tab to only target enemy players, eliminating accidentally targeting pets and totems
It does return the setting to normal upon exiting the above areas)
Very useful when your finishing someone off and a hunter runs into the room and you need to tabstun really fast to finish you work.
Also you may want to get Tidy Plates I've found the shapes of the 'grey' skin make it a lot easier to target via nameplate.