I'm not too sure about the thread "advertising" your services as well. I think that it's just one of the risks reviewers face is that the writer might not even like your feedback. I just keep in mind that I would want anyone to give me ways to improve and hope that it applies to everyone else. If the writer isn't out for feedback, why did they post their story online in the first place?
With a little more detail on the thread, I don't like it. I've been to another site where they do have a thread that people can advertise their services of reviewing, but I applied my story for three of them and never heard back. My main concern is that way too many people would sign up their story and then you'd be swamped with all these requests not all of them would get done. Which, could lead to some bitter members.
To wrap up my thoughts, I think you're just going to have to choose a story that catches your eye and go for it. There's no safe way to tell how someone is going to react. Even if you do make the thread and warn them that you're not completely nice with your reviews, people would say they wouldn't mind, but be offended when they actually do read the review. In other words, some writers might want it until they see it. It's just a risk reviewers have to take.