There's a thing called "prefix tuning" which is basically like a prompt but in a latent space: i.e. prompt which consists of vectors (either key and value vectors, or just input embedding vectors - like custom tokens).

Unlike regular prompts you can optimize them exactly the way you'd do fine-tuning, i.e. if you have examples you can tune your latent prompt to match them as close as possible. I guess the benefit is that you can match style more closely and it would be less artificial. Also they can be rather compact.

Another option is to keep a bunch of LoRA adapters which can be dynamically selected. They can also be very compact.