In the XForms 1.0 specification there is no way to get to the instance element from an instance data node. We have added a function via the getFeature() call on each node, that allows the form author to do that. That is, if instanceNode is a node in an instance document, then:
instanceNode.getFeature("org.mozilla.xforms.instanceOwner", "1.0")
will return the <instance> element (in the main document) that the node belongs to.