Hey Nuts - just have a few moments here, inet is shitty in Venice...
but the bottom line is much like you'd rewrite a url you can rewrite to a proxy. Consider:
RewriteRule ^(.*)$
http://www.someothersite.com$1 [L,P]
although I can't check it ATM, this says that it's the last rewrite, to proxy the request... essentially this makes your server a surfer to the proxied box. I do it in production on a few sites where the front edge is a sun box and it proxies backwards into rendering boxes - I can help when I get home, Sunday.
Hope this points you in the right direction
/p