Mmmm this could be solved using Zero-Knowledge proofs.
When registering pick an elo. Provide a proof that you own the account in that Elo range and then you can create another account that will start in that Elo range.
When registering pick an elo. Provide a proof that you own the account in that Elo range and then you can create another account that will start in that Elo range.
So you get ONE pseudonym and then any other account you create are known aliases of that pseudonym.
And if you play in person, then any account/pseudonym you ever create are linked to your human identity as well.