It is probably easiest to have your slaves assist the master.
And have a key for each of them, to set that target as their Focus and to raid mark the target in the same macro.

Then another key, which goes to all toons to do their CC on their Focus.
Something like:

#show
/stopcasting
/cast [@Focus,Exists,Harm] Whatever

That should interrupt any casting, prioritizing the Focus over DPS.
Not sure if you can include conditions into Stopcasting, such that it only stops if you have a Focus.
The second line will cast the CC at your Focus, but only if you have a Focus and it is hostile.
Pretty much aside from the stop casting, without a Focus your Sheep/Banish/Shackle etc won't fire off.