"document.getElementById('c2nwOpenFileDialog').setAttribute('nwworkingdir', 'your path goes here');"
That will set the open file path.
Make sure it is before you call to open the file dialog.
note: you may want to use the application path or user path provided by the nw.js object expression, to ensure it is correct across all platforms (mac,windows,linux).. since the path will be different for each I think.
edit: I think the app/user path from nw.js object only work when project is exported.