Is there a way I can control what template is rendered in the $Layout field. I know you can create a pagetype but I was wondering if I could do something like
that would control just the $Layout rendering and not touch the page.
Yes you can!
You can pass in an array to the renderWith method where you define templates from specific to unspecific...
This will first look for a template named MySpecialTemplate. If that isn't found, the Page template will be used. This is true for both the main template as well as for the template used for $Layout.
So if you just have a layout named MySpecialTemplate, but no main template, this works equally well. It will render using Page.ss as main template and Layout/MySpecialTemplate.ss for the layout part.