Not sure if youve been asked this, but did you look at the Node Family module for relationship management? http://drupal.org/project/nodefamily

And if you factored it out, then can you add a setting so to restrict the number of links to be created, say by taxonomy terms. If im on the right track i could have ...

category: Role
terms : anon, auth, admin
category: Number of links
terms : 0, 5, 10

use terms in the links system

I hope i'm making some sense