Just from looking at the login procedure for narro I'd say that to support other
authentication backends that some cleanup would need done.  The SSO methods should
behave in a similar method to the OpenID way, but that's all currently hard-coded.

I would think the ideal login process would be:
1) user enters username/password (username could be their openid URL)
2) narro looks up any configured auth providers (OpenID, other custom SSO providers
(based on Zend_Auth_Adapter?)) and passes in the username/password.  Narro could loop
over these until an auth is successful.  If not it could fallback to it's own
built-in users table for auth.
3) if the auth was successful but the user doesn't exist yet, create it similar to
how the openid stuff works now

The code to autentify in Narro against an external database is committed in
http://code.google.com/p/narro/source/detail?r=672 .

Basically I just added an extra way of authentication besides the internal one and
the openid one.

All options are in configuration.inc.php.

I'll keep this open to add a bit of code the actually do the login from another
application.

For mozdev.org, I'm guessing http://code.google.com/p/narro/issues/detail?id=120 is
needed as well.