Possibly relevant to the conversation: Jan Miksovsky (cc'd) had been thinking in this problem space for a while, and has a couple of great blog posts on the topic:
http://blog.quickui.org/2013/11/08/filling-slots-in-shadow/ http://blog.quickui.org/2013/06/11/puzzle-define-html-custom-element-subclasses-that-can-fill-in-base-class-insertion-points/ :DG<
