All i can say is fakecast prefferable with flash heal... if you get it off its still usefull....
If you think Macro's are cheating you are far off from what pvp really is. A macro doesnt do alot of work, but it makes sure that i dont fçk up so much.
Healers can outlast a long time and tab-target doesnt always give the most logical target. Heck sometimes it does nothing since the other target is just outta range...
When playing my shaman or mage i use mousover interrupts, this works perfectly and its a intended use since theres a function for it (target=mouseover)
If your casting something in my range and that cast will heal my target, obviously you get interrupted....