How do I add an EncodingType attribute to the Nonce element of a UsernameToken in WSE 3.0 (.NET)

Question

I'm trying to call a Java Web Service from an MVC3 .NET web app using WSE 3.0.

However, the web service requires an "EncodingType" attribute on the Nonce element of the UsernameToken. Following is a sample SOAP envelope that works correctly with this Java web service:

<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope" xmlns:v1="http://schema.mydomain.org/sms/v1_0">
   <soap:Header>
     <wsse:Security soap:mustUnderstand="true" xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd">
       <wsse:UsernameToken xmlns:wsu="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd">
         <wsse:Username>myUsername</wsse:Username>
         <wsse:Password Type="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-username-token-profile-1.0#PasswordText">myPassword</wsse:Password>
         <wsse:Nonce EncodingType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-soap-message-security-1.0#Base64Binary">XQkp6oYc3DRv41cxkSTW8w==</wsse:Nonce>
         <wsu:Created>2011-09-13T20:50:08.355Z</wsu:Created>
       </wsse:UsernameToken>  
    </wsse:Security>
  </soap:Header>
   <soap:Body>
      <v1:ping/>
   </soap:Body>
</soap:Envelope>

Following is the SOAP envelope generated from the proxy that VS2010 produced (captured in Fiddler):

<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:wsa="http://schemas.xmlsoap.org/ws/2004/08/addressing" xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd" xmlns:wsu="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd">
  <env:Header xmlns:env="http://www.w3.org/2003/05/soap-envelope">
    <wsse:Security env:mustUnderstand="true">
      <wsse:UsernameToken xmlns:wsu="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd" wsu:Id="SecurityToken-111f922b-72c1-4057-bce4-f6555552ce6a">
        <wsse:Username>myUsername</wsse:Username>
        <wsse:Password Type="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-username-token-profile-1.0#PasswordText">myPassword</wsse:Password>
        <wsse:Nonce>qYse3Lor9sAJ9pKPefgkKQ==</wsse:Nonce>
        <wsu:Created>2011-09-13T20:50:38Z</wsu:Created>
      </wsse:UsernameToken>
    </wsse:Security>
  </env:Header>
  <soap:Body>
      <v1:ping/>
  </soap:Body>
</soap:Envelope>

If this attribute is missing, this web service returns, "An invalid security token was provided (An error happened processing a Username Token)"

How do I add an EncodingType attribute?