Why do you need a plugin for something that's simple to set up?
- Create a secondary user group
- Create a user group promotion based on "User has posted at least X messages" and adding them to the secondary user group
- In the specific node, revoke the "Post new thread" permission for the Registered user group (seeing as everyone should have the Registered group as their primary user group)
- In the specific node, allow the "Post new thread" permission for the secondary user group
That's it, it's as simple as that.