I like to use Grid (also need Grid Custom Layout) for click based healing from any window.

You need the addons:
- Grid
- Grid Custom Layouts
- Clique

Grid is the base addon.
Grid Custom Layouts, gets you the ability to do non-standard sorts.
Clique is an addon to interpret clicks as heals.

Open Grid
Go to advanced layouts.
Enter like this, replacing ToonA,ToonB, etc with your toons names.
You want no spaces anywhere.
NAME, NAME, and NR are case sensitive.
All of your toons from all teams, on line one.
Line two gets us toons from the first eight groups, alphabetically sorted, and not repeating anything already done (such as your toons in column one, also alphabetically sorted).

ToonA,ToonB,ToonC,ToonD,ToonE;NAME
1,2,3,4,5,6,7,8;5/8;NAME;NR

Save that.

In IS Boxer Toolkit, you can check the keybind to open the configuration tool in the game.
In the top pane, click the name of your character set.
Ensure there is a check mark in the bottom right pane, on the third option.
"In-Game IS Boxer GUI toggle hotkey".
Note what the hotkey is; I've changed mine to Alt + Ctrl + / as that is what Zanthor had suggested in the original Zero to Hero thread.

Once you're in game, press that key combination (whatever yours is).
Go to the Repeater Region tab.
In the drop down (click the arrow to the right of the target box).
Select the slot that your healer is in.
Check the KVM box.
Then click create repeater region.
Drag the repeater region, so the red (inner portion) covers the entire Grid frame.
It's good to be in a group with your team at this point, so you know the size of the frame.
Once your repeater region covers the desired area, click Sync and then Save it as Auto (which is the default).

Then region swap to you healer toon.
Open up the spell book. "P".
Navigate to your healing tab.
Note there is an extra tab on the spell book now.
Click that, which is the Clique keybinding window.
I like to use Alt + Left click, Alt + Right click, Ctrl + Left click, Ctrl + Right click.
Anyway, mouse over a spell, and click your desired click type.
Thereafter, clicking that mouse/modifier combo, will result in the correct heal type as you just assigned, provided your mouse is over a unit frame.
Close Clique when you're done, binding your clicks.

What we have now.
All your toons are in column one of Grid.
The rest of the BG/Raid is in columns to the right of that.
You have an alphabetical sort, so the Grid is the same for every toon.
You can use the IS Boxer GUI toggle to resize the repeater region (just Sync and Save afterwards) to change it to cover an entire raid if you want.
From any window, you can mouse over the frame of the toon that needs healing.
And will have a fake mouse on the target (healers) screen.
The clicks you have set with Clique will be interpreted as heals on that target.
And it won't change your target on your main/current toon, so won't interfere with your targeting when killing things.