Hello,
I don't think it's possible to get the proxy port from the visitor's HTTP request. Usually, the REMOTE_PORT is set to an unused port on the user's machine (like 57204) or the proxy machine. The proxy port is the one used by the client to access the proxy server. It can be detected however if the user is using a proxy, by analyzing headers like Via, Forwarded etc., not bullet-proof though. So I guess that when a visitor accesses your website, if the request seems to come from a proxy, we could try to connect back to the proxy on a few well-known proxy ports like 3128, 8080 etc. If you want to talk more let me know.
Thanks,
Bogdan