The more I think about this, the more simple it seems and the more useful. ALl you really would need to do is create a new forum and create a moderator with the username/password of the user's account.

Deleting would just require searching for the most receent post and if that post is more than X days old, delete all posts. Obviously that could make problems if someone just has an old board but that's how I would do it (maybe give users 30 or 60 or even 90 days for a receent post).

The hardest thing I can think of would be allowing users to customize the templates for their forum. All the other moderator features are pretty much built in to the actual threads so all of those should work fine.