HTML5 defines a MediaController that is used to coordinate playback of multiple media elements. The current proposal does not support a scenario where a single key is required for multiple media elements coordinated through a single MediaController. One way to solve this would be to create a new interface that provides the Media Element Extensions and then provide an instance of this interface on both the HTMLMediaElement and on the MediaController interfaces. The changes outlined in section Object-Oriented API Design (https://www.w3.org/Bugs/Public/show_bug.cgi?id=16613) might be modified to support this approach.
This isn't a priority for the first release. We should resolve all the issues with HTMLMediaElement before deciding later whether we come back and make this work with MediaController.