Hi Robert,
re: "but how would I be able to hook into that invocation?"
What is the nature of your client? I don't know much about web services, so I don't know what you mean by "webservice client generated by CXF". But I can say that if you don't have direct access to the actual Remoting call Client.invoke(), I don't see how will be able to set the value of the metadata map.
-Ron