If I recall, allowing blank passwords is a setting option, (please correct me if that is no longer correct). An "auto-password" is the same as no password at all in terms of interactive authentication (make sure the local accounts on the computer itself still have passwords however to aid in remote security.)
If you allow blank passwords, and then modify an authorization plugin to only verify the username and return a true response if the username matches your requirements in the authenticate function. If you are trying to use an LDAP server you'll either need a server that allows searching the user ou on an anonymous bind or you'll need a set of LDAP credentials saved in the registry to perform the search with.
pGina will then proceed to the authorization and gateway phases as normal.
If you want the gateway to create a new local account using the provided username and a pre-defined password, you'll either set the password in an earlier phase, or set the password to use in the gateway plugin itself. Having said that, in my opinion, you might as well use a generic account in this scenario since the username/password pair wouldn't match anything else anyway.