I think that you are making this harder than it needs to be
http://www.zend.com/codex.php?id=228&single=1
use that to create a login using cookies on the page... create a folder that is password
Code:
RewriteEngine on
RewriteCond %{HTTP_COOKIE} !^.*access=granted.*$
RewriteRule .*$
http://www.yourdomain.com/loginerr.htm
this code should allow access to the folder if the cookie is set...
http://www.webpimps.com/scripts/htaccess/
taken from this page
then you are good to go
just link to your downloads
edit changed zend link to show auth script with cookie setting enabled