Modifying headers with IIS7 Application Request Routing

I’m using IIS7 Application Request Routing in front of Tomcat as a replacement for ISAPI redirection. The basic reverse proxy function is working well, but I don’t have enough information in the final request headers. My application exists on several subdomains: customerone.ourservice.com, customertwo.ourservice.com, etc. Each subdomain runs the same application, but with different branding graphics.