You could probably hook into the XFormControls ControlsCreated event and browse through the control collection or otherwise maniuplating the data. Or perhaps you can use XFormControl.FormDefinition.SetInstanceData from a BeforeLoadingForm event handler?

Ok, so I am guessing there is no built-in querystring-parsing or other such feature. Coding will be required. Was hoping I wouldn't have to :P

Anders wrote a cool post about extending xform that might help: http://world.episerver.com/Blogs/Anders-Hattestad/Dates/2010/5/Extend-EPiServer-XForm/

Frederik