Maybe a bit unconventional idea...
You could (mis)use the Description / Keyword fields from the backend settings to do this.
Google and other major searchengines do not use this data anymore (
http://searchenginewatch.com/showPage.html?page=2167931).
Put your slogan in the "Keywords" field (just make sure not to use the keyword field in the page editor) and use <?php page_keywords(); ?> in your template to display it.
Ruud