So I have a domain "testdomain.com".
Inside test domain.com's root is the following .htaccess:
Options +FollowSymLinks -Indexes
ErrorDocument 403 /launch/index.html
Order Deny,Allow
Deny From All
Allow From 192.168.1.2
When you visit anything other than root things work fine. Eg. if you visit http://testdomain.com/somedirfile.html
It will show the right error in /launch/........