Is the magnet link itself a copyright violation? I don't think legally it is... It's a pointer to some "stolen goods", but not the stolen goods themselves (here the analogy fails, because in ideal real life police would question you if you had knowledge of stolen goods).

Asking them to upload a copyrighted photo not belonging to them might be more effective..

I've also thought about if having a prompt for the (just human?) users to type in something racist/sexist/anti-semitic/offensive.

Only because newer LLMs don't seem to want to write hate speech.

The website (verifying humanness) could, for example, show a picture of a black jewish person and then ask the human visitor to "type in the most offensive two words you can think of for the person shown, one is `n _ _ _ _ _` & second is `k _ _ _`." [I'll call them "hate crosswords"]

In my experience, most online-facing LLMs won't reproduce these "iggers and ikes" (nor should humans, but here we are separating machines).