Since you can't install mod_layout...try this in .htaccess:
------------------
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !header.php
RewriteRule (.*\.shtml)$
http://www.yourdomain.com/header.php?url=$1
------------------
Then in header.php:
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<?
include("header.html");
include($_REQUEST['url']);
include("footer.html");
?>
------------------
Make header.html your header, and footer.html your footer. That should work. Obviously it's totally ghetto and there is NO error checking
Hit me up on ICQ at 225006418 if you want.