如何在URL重写w/ ARR中获得反向代理服务器的HTTP_HOST ?

I'm using URL Rewrite and Application Request Routing in IIS 7.5 to set up a reverse proxy for a couple of blogs that need to be integrated into existing websites. Multiple domains are bound to one website in IIS, and each one will get a blog that is hosted elsewhere- thats where ARR and URL Rewrite come in. The problem I'm having is that in my outbound ruleset the server variable {HTTP_HOST} pulls the content server's hostname instead of the proxy server's. Is there a server variable that I can use that will give me the proxy server's host hame? Here's a ruleset for one blog with some short comments to clarify:I'm using URL Rewrite and Application Request R