We do have plans to do a Writers Camp specific to existing works someday, but it's going to require a total revamp of the rules and discussion about how to let Tapas readers have a say (if that's even possible in this context with the metrics we have available to us). We're going to do it, but it takes a lot of extra time and resources to make sure we do it right. Right now, we're trying to focus our resources on making Writers Camp as it currently is the best it can be.
There's also a lot of internal discussion happening right now about effective ways to feature self-published creators on Tapas, both for comics and novels. We have a ton of stuff in the works for the future that's going to be helpful for promoting existing novels rather than Tapas novelists relying on Writers Camp for a chance at visibility.
tl;dr: we hear you and we're working on it!