In UVMapper, there are several ways to select specific facets. The marque tool, and the "select group" and select "select material" are the ones I most often use. There is no "invert selection" function that I can see, but with CTRL, multiple materials/groups can be choosen. Choose the facets that you do not wish to work with at the moment and hit "[" . the "hide" key. When you want them back, use the "]" key. To use a layer function, there is an option when you save the template to exclude hidden facets, by the way.