Provide a way that a component sub-class can merge its template with that of its container

This is an idea I've picked up from Wicket; which has a special element, similar to Tapestry t:body, but used to indicate where the child component's template should go. In Wicket, pages are often given a common L&F not by the use of a common component, but by extending a common base class, and mixing the base class' template with the sub-class.