I don’t know of any officially sanctioned way. But, hypothetically, meeting a publicly-known real human person in person and giving them your public pgp key might work. Said real human could vouch for you and your public key, and no one else could fake a message signed by you, assuming you protect your private key. It’s probably sufficient to sign and post one message proving this is your account (profile bio, probably), and then we just have to trust you to keep your account password secure.
I don’t know of any officially sanctioned way. But, hypothetically, meeting a publicly-known real human person in person and giving them your public pgp key might work. Said real human could vouch for you and your public key, and no one else could fake a message signed by you, assuming you protect your private key. It’s probably sufficient to sign and post one message proving this is your account (profile bio, probably), and then we just have to trust you to keep your account password secure.