External Authentication for WPMU


New member here, new to WPMU (though not to WP). I've got a project I'd like to execute and I'm looking for some guidance.

I work for a magazine which has most of its content behind a paywall. We currently run on a custom CMS I built on ColdFusion, but which I'm unable to expand past its current point given my time constraints.

All of our subscription fulfillment management is done through an external company, so I need to have WP authenticate against their database. Unlike most fulfillment companies, however, these guys are tech-savvy, and they provide external-facing webservices for authentication purposes.

My thought was to hack up a PHP script that uses SOAP to authenticate and to then set a cookie. Is there a plugin for WP that, depending on the contents of a cookie, automatically logs in a user, and if the user doesn't exist, creates him? If the user is no longer a subscriber, the plugin would ideally remove him from the WP user database. I'm thinking that this would be the easiest way to keep the two systems in sync.

Please let me know if I'm approaching this incorrectly, or if you have any other thoughts on solutions for integration.