You are correct that the current media handling of he RadEditor dialogs is not using the advantages of the HTML5 functionality. Due to tasks with higher priority I am unable to give a firm response when such matter could be officially planed and implemented.
I can assure you that using the RadEditor with HTML5 players should be supported as expected in all available browsers. If you indeed need a feature that uses HTML5 features to play media in the content of the editor, I would recommend taking advantage of the Editor's availability to create custom tools and dialogs that interferes with the content as implemented. A suitable example about a custom dialog creation is available in this online demo
As for the MediaPlayer control, I can only make assumptions on how do you desire such integration to be done. Note that the RadEditor control is a big and complex control that uses internally plenty of other Telerik UI controls. I assume that using the RadMediaPlayer's Client-side API and a custom user control dialog, you should be able to built a usable feature that provides the desired functionality.
If I am mislead on the required information, please get back to me with more details about the the purpose of such integration and the expected results of such achievement.