I really like this idea, but agree with ntfu2 that it could do with some additional functionality, such as 'random link', etc.
When you say "wish to show to only certain usergroups..." does that mean
a) Only (for example) guests would see the signatures...? Or,
b) Only guest-posts would be forced to have these signatures?
Excuse my ignorance on this matter, but I am not a coder.
Again, a great idea