The plugin documentation and source indicates that the end user will be able to modify the text sent to sending.

 def sending(self, context):

        """

        Suds will send the specified soap envelope.

        Provides the plugin with the opportunity to inspect/modify

        the message text it is sent.

        @param context: The send context.

            The I{envelope} is the envelope text.

        @type context: L{MessageContext}

        """

        pass

Setting context.envelope = 'something else' has no effect on the sent string.  Is this a bug?