<p>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?</p>

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]
```