I had this exact problem using a vb.net client accessing a Java web service.
The solution takes 3 steps:

1.) Create custom ClientOutputFilter
2.) Create a custom Assertion that uses filter from step 1
3.) Bind service to custom Assertion

Step 1
Custom Filter(Called by Custom Assertion)
Class ClientOutputFilter
 Inherits SendSecurityFilter
 Private parentAssertion As UsernameClientAssertion
 Private filterContext As FilterCreationContext

 Public Sub New(ByVal parentAssertion As UsernameClientAssertion, ByVal
filterContext As FilterCreationContext)
   MyBase.New(parentAssertion.ServiceActor, False,
parentAssertion.ClientActor)
   Me.parentAssertion = parentAssertion
   Me.filterContext = filterContext
 End Sub
 Public Overrides Function ProcessMessage(ByVal envelope As
Microsoft.Web.Services3.SoapEnvelope) As
Microsoft.Web.Services3.SoapFilterResult
   Dim securityElement As XmlElement = envelope.CreateElement("wsse",
"Security", "http://schemas.xmlsoap.org/ws/2002/07/secext")
   Dim mustUnderstandAttribute As XmlAttribute =
envelope.CreateAttribute(envelope.DocumentElement.Prefix, "mustUnderstand",
envelope.DocumentElement.NamespaceURI)
   Dim userToken As New UsernameToken(parentAssertion.username,
parentAssertion.password, PasswordOption.SendPlainText)

   mustUnderstandAttribute.Value = "true"
   securityElement.AppendChild(userToken.GetXml(envelope))
   envelope.CreateHeader.AppendChild(securityElement)
   Return SoapFilterResult.Continue
 End Function
End Class

Step 2 Custom Assertion
Public Class UsernameClientAssertion
 Inherits SecurityPolicyAssertion
 Public username As String
 Public password As String

 Public Sub New(ByVal username As String, ByVal password As String)
   Me.username = username
   Me.password = password
 End Sub

 Public Overloads Overrides Function CreateClientOutputFilter(ByVal context
As FilterCreationContext) As SoapFilter
   Return New ClientOutputFilter(Me, context)
 End Function

 Public Overrides Function CreateClientInputFilter(ByVal context As
FilterCreationContext) As SoapFilter
   ' we don't provide ClientInputFilter
   Return Nothing
 End Function

 Public Overrides Function CreateServiceInputFilter(ByVal context As
FilterCreationContext) As SoapFilter
   ' we don't provide any processing for web service side
   Return Nothing
 End Function

 Public Overrides Function CreateServiceOutputFilter(ByVal context As
FilterCreationContext) As SoapFilter
   ' we don't provide any processing for web service side
   Return Nothing
 End Function
End Class

Step 3: Bind proxy to custom assertion

Private WS As New Service.ServicenameWSE
Private token As UsernameToken = New UsernameToken(USERNAME, PASSWORD,
PasswordOption.SendPlainText)
ProvideUsernameToken.Assertions.Add(New UsernameClientAssertion(USERNAME,
PASSWORD))
WS.SetClientCredential(token)
WS.SetPolicy(ProvideUsernameToken)

Good luck, if anyone has a better way to do this I'm all ears

- Ed