I have a few times. I recall seeing a dedication once, a few notes explaining the sources of internal quotes (that they're not violating copyright law, for example, if they come from a song or book), and once a somewhat complicated note explaining that the story was based on a different piece of fiction and was done with the original writer's approval (again, a sort of 'don't sue me'). The actual note I have in mind is a 'this culture may appear to be real but is acutally totally a product of the author's mind, don't hold it as historically accurate' or some such. I remember seeing similar notes in novels, particularily Don Coldsmith's Westerns about a fictional Indian tribe. I'd feel more comfortable stating what I'm doing outright, rather than leaving readers to speculate, since I've had some confusion with betas at this point and it's really not an easy thing to make clear in the story.