I have been all over the Private Messaging function and I've seen no provision for "Global Private Messaging"... but there is no need for that in Private Messaging.
That can be done in an "Update" post that all of your Allies can see on their Home page, and no one else can access.
Click on the Privacy button (default is "Members")
Select "Allies" from the drop-down Privacy menu.
The "Share" icon is removed from the bottom of the post.
Notifications are still active for that post and clicking on the notification still takes you to the Unique URL for that post.
Any of your Allies can still re-post it by copy/pasting the URL, but only common Allies that you share with them can see the post.
If I were to paste the URL into an Update on my Home page and leave it marked as "Members", that post is visible to all Codias members, but if members that are not YOUR Allies were to click on that post, they are directed to a "Log-In" page and not the post marked for "Allies"
One thing to remember though, Nothing is completely Private if even one other member has access to it. Screenshots can be taken of Private Messages AND of Updates marked for Allies only. The same holds true for copying and pasting of text. The best way to safeguard your content that you want to be "Private", is to KNOW YOUR ALLIES!!!!