Instead of delivering according to an engine's agent name, you can also use its IP address. You only need to create and maintain a list of IP addresses.
Anyone who accesses a website leaves his or her IP address, which is resolved to a host name in most cases. For example, a site might visit you during an AOL session, giving the IP 184.108.40.206 (just an example). The web server might resolve this IP address to www.tb03.proxy.aol.com.
If you deliver according to IP addresses, make sure only visitors with the exact IP address can see your page. However, bridge pages may also cause difficulties, if you misuse them.