Again, the problem with forking WP is wordpress.org. yes you can setup a new plugin/theme repository.
The problem is how does someone who owns a plugin on .org validate their ownership on the new repository?
replies(2):
When authors sign up for an account on the new site: have the signup process provide the user a "hash code" to insert anywhere they want into the HTML code or as a HTML comment or header on their homepage to confirm author access to the plugin page on the new site.