> do wonder how hard it'd be to mask the default "marketing copywriter" tone of the LLM by asking it to assume some other tone in your prompt.

Fairly easy, in my wife's experience. She repeatedly got accused of using chatgpt in her original writing (she's not a native english speaker, and was taught to use many of the same idioms that LLMs use) until she started actually using chatgpt with about two pages of instructions for tone to "humanize" her writing. The irony is staggering.