I just installed the plugin "RSS Footer" and tested it. Works fine on 3.0.1, except that its apparently not UTF-8 compatible. If you try to save characters on the options page which are not ISO-8859-1 (like e.g. German umlaut characters), they will show up broken after saving and in the feed. This is easily fixed though:
Open rss-footer.php and find the end of line 57 where it says:
Change this to:
.stripslashes(htmlentities($options['footerstring'], ENT_QUOTES, get_bloginfo('charset'))).