I realize the question's title is about powershell, but not really what the question is asking? Either way...
The WebClient is a very dumbed down wrapper for HttpWebRequest. WebClient is great if you just doing very simple consumption of services or posting a bit of Xml, but the tradeoff is that it's not as flexible as you might want it to be. You won't be able to get the information you are looking for from WebClient.
If you need the status code, get it from the HttpWebResponse. If you were doing something like this (just posting a string to a Url) w/ WebClient:
var bytes =
System.Text.Encoding.ASCII.GetBytes("my xml");
var response =
new WebClient().UploadData("http://webservice.com", "POST", bytes);
then you'd do this with HttpWebRequest to get status code. Same idea, just more options (and therefore more code).
//create a stream from whatever you want to post here
var bytes =
System.Text.Encoding.ASCII.GetBytes("my xml");
var request =
(HttpWebRequest)WebRequest.Create("http://webservice.com");
//set up your request options here (method, mime-type, length)
//write something to the request stream
var requestStream = request.GetRequestStream();
requestStream.Write(bytes, 0, bytes.Length);
requestStream.Close();
var response = (HttpWebResponse)request.GetResponse();
//returns back the HttpStatusCode enumeration
var httpStatusCode = response.StatusCode;