As far as properties being passed up to including templates, I think this is a great idea. I can think of many uses this would have had in past projects.
One difficulty that you didn't mention: If the included template was also meant to be a stand-alone page, most of the time it would have a <master> tag as a part of it. The templating system would have to be smart enough to remove or ignore that tag to avoid interfering with the including page's <master> tag.
