eval is HTTP:response

You can definitely use evaluated expressions in an HTTP::respond command, but your example is a little off. You would include any HTTP headers AFTER the content. Example:

when HTTP_REQUEST {
    HTTP::respond 200 content "..." "True-Client-IP" [IP::client_addr]
}

Your example assumes the X-Forwarded-For header would be in the HTTP request, which the client (a browser) would not generally send. So if you need to send the client source address back to the client, you could simply use [IP::client_addr].