I'm quite surprised that this hasn't yet been thoroughly addressed, as it's something that I ran into fairly quickly.
My site structure looks something like this:
- Company ->
- Portfolio ->
And so on. It's a three-level hierarchy. All of the subpages are very simple, and don't need their own controller logic. They're simply template content that wouldn't do well with being in the CMS.
I would like to have these subpage templates in a subdirectory of Layout, and rendered automatically within their parent pagetype template. I cannot figure out how to extend the template selection of SiteTree to do this.