Unfortunately, MediaElement does not support the Stretch property like it did in Silverlight; therefore we don't support this in MediaPlayer either.
It is possilbe that we might someday add this by "faking" the feature using ScaleTransforms and cliping but need to further evaluate the performance trade offs first. For example: GPU rendering may not get triggered in this scenario and therefore
it may not be recommended.
For now, you could technically do this yourself by finding the MediaElement in the VisualTree if it is a requirement for your app.