Difficulties I can think of:
* Getting ai companies to offer this. I don't think it comes with a downside for them really though. You couldn't use it to actually retrieve results in any way, and you wouldn't have prompts.
* This only detects exact matches, people changing the output would defeat it. Fixable with some kind of fuzzy search that returns a 'distance to nearest response' but this obviously makes it more expensive and difficult to run, use, etc.
* People could still run the model themselves, as models get better and more expensive maybe this becomes less of a problem. Or maybe models get small enough while still generating good output that it becomes more of a problem. Who knows.
At least this would avoid AI grading AI issues