An interim fix (player side) for off-tank assisting or crowd control could be another boxing assist method.
We have FTL Assist.

Meaning we don't need to use our Focus, Party Leader or Target for assist.

You could have the target of FTL Assist Me, be an Action Target Group.
Have a toggle to add/remove certain toons from this group.
So they assist normally when in it, but not when you take them out of it.

Then use.... Their target as their CC target.
Meaning, have the slave assist you once to acquire their CC target.
And then hit their toggle to take them out of the FTL Assist Group.
So now they have assisted you, acquired their CC target.
You could probably use an isboxer warcraft macro action, which assists FTL and then returns to original target.
And have them cast (recast) their CC at their own target, with a harm conditional.
Something to that effect.
Once the CC target is done, have a mapped key to add all characters (or a toggle per toon) that adds targets to the regular FTL Assist action target group.

Which leaves your Focus for secondary assisting.
Give them a warcraft macro (or isboxer warcraft macro).
Where if they have a focus and it is friendly, that is their assist.... /assist [@Focus,Exists,Help].
Then whatever their spell or castsequence is for that hotkey.
If you give them a Focus, ie off-tank to assist, then even though IS Boxer will have them target the main tank's target the macro will direct the nuke to the off-tank's target.
You could have them Follow their focus, if it is friendly too.
When they don't have a Focus, or the Focus is hostile, they won't assist it.

If you never had CC targets and Off-Tank assisting in the same fight.
You could probably use Focus for each, with [harm] and [help] conditions.