I was wondering if it was possible, through macros or addons, to have a button that will make the main cast Hammer of Justice if it isn't on cooldown, otherwise have slave #1 cast it, otherwise have slave #2 cast it, etc. That way, only one character would cast spells like Hammer of Justice every time instead of wasting the ability on all the slaves.
Either set up a round robin in your key broadcaster, or a round robin macro.
It's not possible in the way you are asking, but in practice, you can get it to work that way with the following cast sequences (assuming 5 characters here, set the reset to whatever the cooldown is for you).
Main:
/castsequence reset=60 Hammer of Justice,,,,
Slave1:
/castsequence reset=60 ,Hammer of Justice,,,
Slave2:
/castsequence reset=60 ,,Hammer of Justice,,
Slave3:
/castsequence reset=60 ,,,Hammer of Justice,
Slave4:
/castsequence reset=60 ,,,,Hammer of Justice
Macros cannot make decisions. You can use round robin broadcasting or cast sequences to stagger usage, but it is not possible for a macro to look at your spells and decide who should cast.
i use round robin macros all the time. when running multiples of the same class these types of macros effectively give you a shorter cooldown on your big ablities. Shamans thunderstorm, death knights death grip, boomkins typhoon, etc.