I think this might be something that works better as a canon-specific DNW than a general. As people have mentioned up thread, if I saw DNW canon pastiche for, like, Victor Hugo or Hidden Almanac or Douglas Adams, that would be fairly clear and reasonably enforceable (I'm still not sure that mods would enforce it for something like wrote a humor fic for a humor canon, and you might be better off dnw'ing specific aspects, but at least it would be a reasonable guide for the author.) But for most modern book fandoms that don't have a super distinct style, idk how a writer would even do it. Write the characters OOC? Write a modern book fandom in Dickens style? (Plus there are fanwriters with a really distinct voice of their own. Do you just not want something that sounds like canon? Or would you also not want something super-stylized its own way?) And if you're requesting visual fandoms it makes no sense at all.

Also yeah I remember a phase in fandom when "pastiche" was used to mean "untransformative badfic" and sadly you can't dnw that no matter how you word it.

tl:Dr if this dnw is only on book fandoms with a super distinct canon style/form it's probably fine to dnw "canon pastiche", otherwise be more specific. DNW all pastiche is technically banning your writer from using any style or form which is tough.