Also, I'm sure that you can eventually just prompt the model with the message you want to put into the story, if you can't already do that.
But that wouldn't be helpful. It would've been memorable, because novels aren't just about the message.
AFAICT current text generation is something approaching bad mimicry at best and downright abysmal in general. I think you still need a very skilled author and meaty brain with a story to tell to make use of an LLM for storytelling. Sure it’s a useful tool that will make authors more effective but we are far from the point where you tell the LLM “write a story set in Pratchetts Discworld” and something acceptable or even entertaining will be spit out - if such a thing can even be achieved.
Given all that, the purpose of LLMs should be to create tailor made content to everyone's tastes. However, it seems the hardcore guardrails put into GPT4 and Claude prevent it from generating anything enjoyable. It seems, even the plot of the average Star Wars movie is too spicy for modern LLM sensibilities, never mind something like Stephen King.
I'm sure OpenAI's models can shit out an approximation of a new Terry Pratchett or Douglas Adams novel, but nobody with any level of literary appreciation would give a damn unless fraud was committed to trick readers into buying it. It's not the author's work, and there's no human message behind it.