If you're talking about anything linked to your wallet as a proof of identity, other than for said wallet without your actual identity, I'm out.
Discussion
See if you like this, Bitcoin as an authentication mechanism:
You have a utxo in an address you control.
A gateway, preprogrammed with that address as your identity, gives you a challenge token and you sign it with that address' keys.
Or, if for whatever reason, you can't send a signed message to the gateway, it can be preprogrammed with your xpub as your identity. You obtain a challenge token and put it in a transaction that moves that utxo to the next address in your xpub, it gets mined in a block, and you obtain access via the gateway.
