Make the usernames "username@domain.tld". If the user is already logged in the button will act normally. If its not you simply ask them to log in. If they use

Make the usernames "username@domain.tld". If the user is already logged in the button will act normally. If its not you simply ask them to log in. If they use username@persona.com on personb.com, handle that in the login code and start the regular OpenID flow with persona.com to authenticate. personb.com is now allowed, on behalf of the user, to reshare this content on persona.com, and sends the data directly to an endpoint there.