There is another way you can use IP blocking with your htaccess that will give a 403 error if the user is not on the IP secified. Then rather than using the standard 403 document you can redirect them to a different one with the following command.
Then in that page 'newerrordoc.html' you can put an html redirect command
ErrorDocument 403 /newerrordoc.html
Oh here's the IP blocking htaccess code
<meta http-equiv="REFRESH" content="0;url=http://www.the-domain-you-want-to-redirect-to.com">
Another alternative would be looking into using apache mod rewrite. If the above method doesn't work i'll look into that for you.
<Limit GET POST>
deny from all
allow from 188.8.131.52