I don't know if this is supported. My guess is that it isn't for hosted InfoPath forms. If you have a handle to the InfoPath instance, you could write a Web service on the server to get the parameters and set a couple fields in the InfoPath form. Usually for passing parameters we use import via the command line /aggregate switch or we use a sample xml file and NewFromSolutionWithData or something like that. I'll ask David to respond. He worked on Form Server...
Patrick Halstead
Project Manager at Qdabra