zaus on "[Plugin: Contact Form 7] Add solution for 3rd party integration and managing errors"

I'd suggest looking at how my plugin http://wordpress.org/extend/plugins/forms-3rdparty-integration/ hooks to the wpcf7_before_send_mail action to alter the $form->messages['mail_sent_ok'] (screen message) or $form->mail['body'] (email). Main point is never alter plugin code if they provide hooks, and CF7 is good about providing a lot of hooks.