Unfortunately XPath does not provide any sorting functionality (in the XML ecosystem, XSLT and XQuery do). However, eXForms has a specification for a sort function here. Orbeon Forms supports the eXForms function as well as a slight variation in their own namespace.
It would seem reasonable to support one of those. I haven't thought through the implications of the slight difference between the two. I'd recommend that the next step be to identify someone who can build the functionality. That person can then make a formal proposal.