I had another idea about adding an "Exclude Usergroups" option. Not sure if you'll like it as much as my "Exclude Forums" idea, but I thought I'd suggest it anyway.
I just thought that this might get pretty server-intensive if all guests can use it, so I'd like to make it an incentive to sign up! And that's another thing, dbirosel, I can't PM guests to tell them about it, so I might as well just not let them use it.