This is more or less the way SAML [1] works, at least in one of it's protocols. The authentication server redirects (after successful login) to the initiating service and supplies an assertion-token, often with limited validity. With that assertion token more data can be asked from the servers to the auth-server directly. No account_id's go over the wire, the cookie needs only to be valid on the authentication server. And you should really need it, it's not much fun to implement.

1 http://en.wikipedia.org/wiki/SAML_2.0#Artifact_Resolution_Pr...