Middleman can be used to transparently proxy requests; to make use of this feature, you will need to use firewall software capable of forwarding connections. Configure the firewall to forward connections destined for port 80 to the proxy server; the proxy server will look at the Host header sent by the browser and use that to determine what host the request was originally intended for. This feature may not work for all browsers, sending the Host header is only required for HTTP 1.1, although most HTTP 1.0 clients send it anyways.
If you are using Linux as a firewall, the following iptables command will transparently proxy all outgoing requests on port 80 (replace eth0 and 8080 to match your setup):
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport http -j REDIRECT --to-port 8080