This can be done by making modifications to the CSS files in your customization. However, this probably needs to be carefully designed so that you do overwhelm your authors with too much information. See:
http://forums.xmetal.com/index.php/topic,925.0.htmlAn alternative (in case you are not aware) is to hover over the tags of an element. When you do this XMetaL will show you the values for attributes that have values.