Not sure if I'm oversimplifying your request, but..
as long as allowoverrides is turned on for the user's homedirs, a user can use rewrite rules in their .htaccess. e.g. one of my .htaccess files on my shared webserver looks like:
Code:
RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://(www\.)?myserver\.com/.*$ [NC]
RewriteRule \.(gif|jpg)$ http://myserver.com/test/createImage.php [R,L]
... and that exists in only one subdir of my site, all others operate normally, i.e. no rewrite rules.