> Throw a pregenerated cert into localStorage and sync across browsers.
Browser extensions use their own secure storage mechanisms, not the localStorage used by sites, but yeah.
> Where I'm still fuzzy is if the site performs local verification, or login.persona.org does it, it might contact your server to grab its pubkey over SSL, requiring you to host something on your email domain.
Ignoring the workflow for domains without native identity providers (for which persona.org does its own mail-you-a-link validation), only one thing happens on the server accepting Persona: it requests the public key from the email domain. The site then expects to receive a message from the browser that chains back to that public key. So, you'd have to post some static files to /.well-known/ on your email domain, but the browser extension would construct the signed messages because it has the private key.
You could do this in two ways. For domains completely owned by the user, you could let the browsers hold the domain private key, and mint arbitrary identities on that domain on demand. For domains with multiple users but still wanting to statically manage identities, you could statically generate public/private keys for users and sign them with the domain key, then let the browsers hold onto the email keypairs only.