Você deve usar a opção 'P' (proxy) na regra mod_rewrite para ser feita internamente no servidor. Você também precisará ativar o mod_proxy .
dos documentos do mod_rewrite :
'proxy|P' (force proxy)
This flag forces the substitution part to be internally sent as a proxy request and immediately (rewrite processing stops here) put through the proxy module. You must make sure that the substitution string is a valid URI (typically starting with http://hostname) which can be handled by the Apache proxy module. If not, you will get an error from the proxy module. Use this flag to achieve a more powerful implementation of the ProxyPass directive, to map remote content into the namespace of the local server.
Note: mod_proxy must be enabled in order to use this flag.