There’s now an MSE post allowing us to opt in, which is probably the best way to do this unless we want something custom for some reason.
Sites can now request to enable a banner to warn about their policy on AI-generated content
Once we have consensus, a mod can give this question status-reviewstatus-review.
Per this:
Both options have a "Learn more" link, which will point to a per-site help center article, whose contents should also be a part of the community discussion — this article should explain what the site's policy on AI-generated content is (here's SO's article, as an example).
It looks like we might need a help center page about our LLM-usage ban, too. IDK if we're allowed to link to Meta instead.