Another plugin I am using (that moves images to Amazon S3) is hooking into the ‘wp_update_attachment_metadata’ filter. This works quite well but probably is a bit quick and dirty.
The plugin doesn’t work (I’m using WP 2.8.6) because the invitation code isn’t passed as a field in the second form at registration. This can easily be done:
In the initialization routine, add add_action( 'signup_hidden_fields', array( &$this, 'add_hidden_field_to_signup' ) );