[Erwin]

I have not installed this hack nor have I tested it, but surely you can just do this to allow mods and admins (or any other usergroup the ability to bypass this):

In functions.php, find:

if ( (in_array($bbuserinfo[userid], $uids)) and ($allowbuddypm) )

Replace with:

if ( (in_array($bbuserinfo[userid], $uids)) and ($allowbuddypm) and (!in_array($bbuserinfo['usergroupid'], array(5,6,7))) )

Change 5,6,7 to any usergroupid you want that will always be able to PM.

Again, I repeat, I have not installed this hack nor tested this.