I had a thought... could we leverage this model as a means to access device-specific data? eg have a 'special' instance name - instance('device') - which could expose device metadata/settings; eg deviceid, subscriberid, simserial, ...
Other XForms clients (eg Survey123) appear to expose this stuff via a custom XPath function (
property(...)). But there seems to some appeal to leveraging this new, universal, XPath external instance technique (with full XPath query support as a bonus) to access 'external data' - which arguably this device metadata is.