There are a couple of approaches to this.
1. Use the API to set the thread author
This endpoint is used to update a thread - you'll need the author's Disqus username and the Disqus unique thread ID:
http://disqus.com/api/docs/threads/update/
You can acquire the Disqus thread ID using a custom identifier or URL using this endpoint:
http://disqus.com/api/docs/threads/details/
2. Send the notification manually
We have an example available which shows you how to do this using our js callback and a PHP script:
https://github.com/disqus/DISQUS-API-Recipes/tree/master/notifications