By using a browserId enabled user agent/browser and an email provider who has implemented the browserId -protocol you already have a fully decentralised Persona experience.
My problem is with the "email provider" part. In the real world, it just means Google, Yahoo, Microsoft, and handful of firms (like Rackspace) that provide hosted email solutions for a fee. You might call it "decentralized", but I'd call it "mostly centralized".
By using a browserId enabled user agent/browser and an email provider who has implemented the browserId -protocol you already have a fully decentralised Persona experience.