[userbinator]

Not if you receive the HTTP request from the client first, before any interaction with the end-host.

[JDye]

If the proxy can "see" the requests, then this isnt an issue because the headers can be trivially be modified.

The problem is that the proxies which are targets of identification - think proxies for large scale web scraping which use CONNECT tunnels - dont get to "see" the request.