As Stefan points out it is not possible to use this module to do as you require. However if the problem can be summerised as when a user logs in they should only see the data taken from a web service which they maintain, then an alternative to writing a custom module would be to
1) Wait for the XML module to include tokens (subject of this thread) which will be by the end of the year hopefully for the official XML module. Then wire up the XML source to be ..../Getdata.aspx?User=[USERID] or similar
2) Write a simple ASPX page on one of your servers (GetData.aspx) which will take the parameter and match up the userid with the web service. Invoke the required web service to get the xml back then return this data using response.write etc etc
i.e. Just code the plumbing between the module and your set of web services