I use Keybase to cryptographical prove my identity across sites. However a bot would not need to be so complex, just issue a random string or pass phrase the person would need to post on the other site. There could still be problems - sometimes a person's content be it words, photos, audio or video is already plagiarized but the bot doesn't know which site is the original one that an author should claim.
Mostly I think that bots should always have a complaint mechanism to their human controller. If they can't respond to requests for review add redress of wrongful actions a bot should be banned from the system IMO
That's not a bad idea...