you can't do this directly only by an addon, because you need to trigger the heal somehow on your healer...

..what i'm doing for all my group heals is:

1. I use grid to show all my chars on my main in alphabetic order
2. I use this grid-unitframes as graphical layout for my healing buttons
3. When I click on such a virtual healing button, i broadcast a specific key-combo which identifys the target and which heal to use to my healer (like an ftl button)
4. my Healer has on this key, an ftl-style healing macro

To make this work as I said I use Grid and HotKeyNet.

To get an Idea who this could be done with hotkeynet, the corrsponding parts on my script look like this, for activating a Greatar Heal by left clicking:

Code:
<Hotkey LButton>
 	<If MouseIsOverWindowRect WoW1 900 600 40 40>  //This means its on window WoW1 (which is my tank) over the first grid entry (which is CharA)
           	<Sendlabel healer>
                 	<Key rctrl o>
 	<Else If MouseIsOverWindowRect WoW1 940 600 40 40>  //This means its on window WoW1 (which is my tank) over the second grid entry (which is CharB)
            	<Sendlabel healer>
                  	<Key ralt o>
 	<Else If MouseIsOverWindowRect WoW1 980 600 40 40>  //This means its on window WoW1 (which is my tank) over the third grid entry (which is CharC)
            	<Sendlabel healer>
                  	<Key rshift o>
 	<Else If MouseIsOverWindowRect WoW1 1020 600 40 40>  //This means its on window WoW1 (which is my tank) over the fourth grid entry (which is CharD)
            	<Sendlabel healer>
                  	<Key lshift o>
 	<Else If MouseIsOverWindowRect WoW1 1060 600 40 40>  //This means its on window WoW1 (which is my tank) over the fifth grid entry (which is CharE)
            	<Sendlabel healer>
                  	<Key lalt o>
 	<Else>   //To pass the mouseclick, if no region was hit
         	<SendFocusWin>
                	<ClickMouse>
with the following macro on my healer bound to 'o':
Code:
/cast [mod:rctrl,target=CharA][mod:ralt,target=CharB][mod:rshift,target=CharC][mod:rlshift,target=CharD][mod:lalt,target=CharD] Greater Heal
Of course, if you would get Healbot to sort alphabetical (and when you find this option, please say so!), you could simply position it the same on your tank and heaer and broadcast the mouseclicks...