The recommended way would be to do this in Xaml by creating a custom template for the controlpanel. After doing this, to change the slider's position and characteristics by Timeline control's properties.
As an example of this, you can also refer to the EntertainmentTheme.xaml file shipped with the player framework for an example of altering the default style of the player. Also look at EntertainmentAppPage.xaml in the samples to see how to wire this up to
Hopefully this helps, Tim