Is there a possible htaccess directive that can transparently forward request from index.php to index_internal.php if the request is coming from an internal ip range?
Something like this should do it (obviously change the IP address to match your network):
RewriteCond %{REMOTE_ADDR} ^192\.168\. RewriteRule index.php index_internal.php
If you want an actual header then make it RewriteRule index.php index_internal.php [L,R,QSA]
RewriteRule index.php index_internal.php [L,R,QSA]