Query about using PostURL with PB10.2

I need to pass a SOAP instruction from a PB application to a 3rd party product, including a Word document.   Using base64 encoding and geturl, I can do this, but it is very slow and our suppliers recommend passing this as a binary stream.  They are unable to provide much assistance with their API, but have provided some Java code, that they say does the trick, as follows...
      public static String rkyvCallWithStream(String sSoap, byte[] data) throws Exception
      {

            Vector results = new Vector(3);
            StringBuffer rtn = new StringBuffer();
            String inputLine;
            BufferedReader in = null;

            OutputStream out = null;
            HttpURLConnection connection = null;

            try {

                  String urlString = "http://" + RKYVMACHINENAME + "/servlets/servlets.CH_CherryServlet?Soap_Message=" + sSoap;
                  URL url = new URL(urlString);
                  System.out.println(urlString);

                  connection = (HttpURLConnection) url.openConnection();
                  connection.setRequestMethod("POST");
                  connection.setRequestProperty("content-type", "text/xml");
                  connection.setUseCaches(false);
                  connection.setDoOutput(true);

                  out = connection.getOutputStream();

                  System.out.println("Writing binary array...");

                  out.write(data, 0, data.length);
                  out.flush();
                  out.close();

                  in = new BufferedReader(new InputStreamReader(connection.getInputStream()));
                  while ((inputLine = in.readLine()) != null) {
                        rtn.append(inputLine);
                  }

                  in.close();
                  connection.disconnect();
            } finally {
                  try {
                        out.flush();
                        out.close();
                        in.close();
                  } catch (Exception e) {
                  }
                  connection.disconnect();
            }
            return rtn.toString();
      }
I've had no joy in getting Powerbuilder to do something similar using PostURL.  I suspect that Java is doing something hidden in the way of headers etc.
Our suppliers say they've never heard of anybody using this calling with non-Java clients and helpfully suggest that we recode our application in Java.
Can anybody offer any other suggestions on this?  We are running PB10.2 and don't use PFC or EAServer