In that case, just retract _any_ part of the key, such that it won’t work if you copy paste it, but still no used secret is revealed.

If a user wants to rebuild the key in order to use it for something, then they must have a very good reason to do so, and it is probably better to just let them do that.

While I understand the sentiment, like all attempts at outsmarting stupid people, it is misplaced. If they're not copying your fake private key they're just copying someone else's.