We already have such a feature request. It is logged in our public issue tracking system and you can track for its status from here: http://www.telerik.com/support/pits.aspx#/public/silverlight/3751 . You can vote for it in order to increase its priority over other tasks.
However currently the media player does not support such functionality out of the box. You should change the style of the player (more precisely - change the style of the RadMediaPlayerWrapper) and customize it to fit your own needs. Also you can check this article about restyling: http://www.telerik.com/help/silverlight/common-styling-appearance-edit-control-templates-blend.html
Hope this helps. Let me know if you need more info.
the Telerik team
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