Crossfingers, Khaz'goroth taken from wow forums
Boy, it sounds like you are a very visual healer! Now, I know for a fact there isn't an addon for this and I'll tell you why: Healing is multidirectional. For the most part you can cast any healing spell in any direction without worry about where on the map the players are. Not only that, but with large raids, 40 HP windows in a very small area would be a LOT of information to pile onto that mini map! If you're having problems with people walking out of range make sure you set your healbot or vuhdoo (my personal favorite) to fade out or even highlight the player who is out of range! Vuhdo will even put arrows up to tell you the direction of the out-of-range toon!