As far as a round-robin mapped key goes.

Create your new mapped key.
Give it a hot key to activate.
Give it four steps, since there are four toons.

Step 1, target is Toon A
Step 2, target is Toon B
Step 3, target is Toon C
Step 4, target is Toon D

The easiest is to just send a keybind to warcraft on each step.
With your macro bound to that key.
The advantage is that each toon could one-box, and the hotkey is what you're used to and works 1-boxed or multiboxing.

You can also do macros inside of IS Boxer.
If you do this, you need a key combination, including modifiers.
An advantage is 1024 characters instead of 255.
And you can use arguments like {FTL}, etc.
I've not done too many macros inside IS Boxer, but any that you do make are easy to transplant to your next team etc.

If the wow macro is placed on one of your FTL DPS keys, the slaves should auto assist the active toon.
And in turn (round-robin) their Repentance.
If its not on an FTL DPS Key, it's pretty easy to add:
Do Mapped Key Action - General - FTL Assist Me (target Current Window).
And then the hotkey for the wow keybind.





Not sure on the Vuhdo thing.
I use Grid + GridCustomLayouts.

But I'm quite sure you can do this in Vuhdo.
It seems to be very flexible.

In Grid...

Type /Grid Config.
Click Layouts.
Click Advanced.
Press New.

Enter: nameList="ToonA,ToonB,ToonC,ToonD,ToonE"

Click Save.