Well, yeah. How would you even do a reverse CAPTCHA?
There's a little hint of this right now in that the "reasoning" traces that come back from the JSON are signed and sometimes obfuscated with only the encrypted chunk visible to the end user.
It would actually be pretty neat if you could request signed LLM outputs and they had a tool for confirming those signatures against the original prompts. I don't know that there's a pressing commercial argument for them doing this though.