I think that the scatter-gatherer pattern would work well for this. In your orchestration split the clients xml document into individual client messages, then publish these to a orchestration that handles them along with a correlation token. The orchestration that does the individual calls and sends the response back to the controlling orchestration. An example can be found at http://www.richardhallgren.com/a-loosely-coupled-scatter-and-gather-implementation-in-biztalk-2006/
Another place for an example of this is at http://blogs.msdn.com/b/cvidotto/archive/2006/12/18/biztalk-server-2006-code-samples-awesome.aspx. Search for Implementing Scatter and Gather Pattern.