Currently the template engine does not provide anything to allow sorting by a custom field. It would be possible only by using the list to build an array with the resulting articles and then processing that list from the list with some PHP code to provide the sorting and render the list. Neither ideal nor optimal but it would work.
I know there's a Newscoop user who is working on providing this functionality, but we cannot tell you if it will be merged onto the core until after a proper evaluation and testing. We will keep you posted here in this same thread about it.