I can think of a fairly easy way with IS Boxer.
But not sure that you are using IS Boxer or not.

Create your mapped key.
Have it look something like:

Hotkey: Whatever
Steps 1:
- Do Mapped Key Action (Focus Heal - mapped key)
- Do Mapped Key Action (Normal Heal - mapped key)

Then create another mapped key
Which toggles on/off the Focus Heal and Normal Heal mapped keys.
One is on, and one is off.
Press it again, and this reverses.
You could give more then one shammy this toggle mapped key, each on their own keybind.
And even have the mapped keys (Focus Heal vs Normal Heal) enabled or disabled on a per toon basis.