I think this may be my struggle with the file based templates. Details of my adventures on this are here: http://www.sitefinity.com/developer-network/forums/project-feather/new-file-based-templates
You can add new files to /GridSystem/Templates and they'll show up in the layout widgets. The new layouts have to be put into /MVC/Views/Layouts. Once there, you'll see a new mylayoutname.cshtml file in whatever your resource package folder name is.
The problem that I outlined in my post, is that this works fine but there's no idea of building one template from another so I can't drop in template1.cshtml and then template2.cshtml that builds on template1. It has to be a full page template from what I can tell.