I haven't even heard of one. Do you not think if it was a realistic potential, one of the two of us would have some kind of practical experience by this point? I think we can preclude it at the current state of technology.

Moreover, I think we can preclude it not just on empirical grounds but also theoretical ones. Fiction is meaningful to us because it communicates something. No one is interested in hearing their friends' pointless dreams every morning. Generative agents cannot communicate because they presently do not have a point of view, a theory of self, and cannot have experiences; they have nothing they can meaningfully communicate. A novel crafted for you by a glorified Markov chain can at most only ever be a mirror to see yourself. Which can be interesting and have merit, but it's a totally different thing that just happens to be (potentially) novel-shaped, it's not a novel.