Hi Tim,
As far as I know, Windows.Media.Playback.MediaPlayer is currently relying on XAML or Composition API, which are not available in html.
So, it is not possible to use a Windows.Media.Playback.MediaPlayer in UWP(JS) together with video element.
If you still have requirements on this feature, you can try to submit your idea to User Voice site: https://wpdev.uservoice.com/.
You can also submit this feature request/feedback in the Windows 10 feedback app.