Hi Adam,
Yes, this is the limitation of using a XML file as a 2DS (secondary data source). You could of course, have an InfoPath solution on top of that file and change the main form to automatically "import" this secondary resource data file on load from a domain trust web location... Or, you could use our database accelerator to accomplish the same thing and dispense with saving the file on a web site. Lots of options here... :)
Patrick Halstead
Project Manager at Qdabra