Right, you have the same zen 1.3.7 like me. Nothing can be done about the homepage tags when editing the meta-tags.php. You have to edit the html_header.php, thats the only way. But make sure you made a backup.. there is the line:

<meta name="keywords" content="<?php echo META_TAG_KEYWORDS; ?>" />
and under that the same line with : "description", you have to replace both lines with code.