Law professor Mark Hall takes a stab at it (in the title of an otherwise commendable piece about the bogus legal claims of the states challenging the constitutionality of the Affordable Care Act) with the ill-considered phrase “Attorneys General’s.”
Now, I’m inclined to think there’s not a right answer here and that there are a few ways you could go. But I’m pretty sure that “Attorneys General’s” is not one of those ways. …
Part of the problem is that attorney general is not a phrase of English origin in the first place. Like many other legal phrases, it came into English from the French following the Norman invasion. The general part was originally an adjective modifying attorney. (In French, the adjective can come after the noun.) An attorney general might just as well have been called a “general attorney.” Think of it as meaning something like “top attorney.” We would have no problem talking about top attorneys or top attorneys’ arguments, but we would stumble all over attorneys top.
Another part of the problem is that possessive forms of title phrases are often clumsy. When the President of the United States has a plan, it’s the President’s plan—not the President’s of the United States plan, or the President of the United States’s plan. But the latter might not sound so silly in some contexts. Say, at a global summit, where there are lots of presidents. Or think governors. It doesn’t really sound weird at all to talk about the Governor of Georgia’s plan.
There’s so much more. It’s worth a read, if you care about such things. (And I know you do, deeply. As you should!)