I just checked with a sample DjVu file: we can make thumbnails of them using ImageMagick, and those can be displayed. If you can otherwise make thumbnails but not of DjVu files, it’s possible your installation of ImageMagick just doesn’t have DjVu support (or has it disabled).
As for how UniversalViewer would work with them, I can’t really say. Maybe someone else knows more about that.