Você pode usar o RewriteRule para fazer proxy de uma solicitação. A documentação do apache para usar mod_rewrite como proxy mostra o seguinte:
Description:
mod_rewrite provides the [P] flag, which allows URLs to be passed, via mod_proxy, to another server. Two examples are given here. In one example, a URL is passed directly to another server, and served as though it were a local URL. In the other example, we proxy missing content to a back-end server.
Solution:
To simply map a URL to another server, we use the [P] flag, as follows:
RewriteEngine on
RewriteBase "/products/"
RewriteRule "^widget/(.*)$" "http://product.example.com/widget/$1" [P]
ProxyPassReverse "/products/widget/" "http://product.example.com/widget/"
In the second example, we proxy the request only if we can't find the resource locally. This can be very useful when you're migrating from one server to another, and you're not sure if all the content has been migrated yet.
RewriteCond "%{REQUEST_FILENAME}" !-f
RewriteCond "%{REQUEST_FILENAME}" !-d
RewriteRule "^/(.*)" "http://old.example.com/$1" [P]
ProxyPassReverse "/" "http://old.example.com/"
O segundo exemplo, reescrevendo condicionalmente URLs com o sinalizador [P] para representar o pedido, parece o que você precisa.