Hi RX,
If you want the client to see the change in the address bar, you'd want to use a different iRule that sends a 30x redirect. ProxyPass by design hides the rewriting. If you want the traffic for an external site proxied through your virtual server you'd need to rewrite the requests like ProxyPass does rather than send them a 30x redirect. To use ProxyPass for an external site, just set up your VS default pool for the server IP address(es) that host the remote content. You'll need a route on LTM for the remote servers and SNAT on the virtual server. If you have multiple pools which serve the content, you can specify the pools in the ProxyPass datagroup. You can check the comments in the ProxyPass rule for details on these points.
Aaron