I'm sure there should be a better way than this, but you can just use the special character directly in xaml.
I managed to cut and paste the back arrow into the contents of the text block, and it displayed correctly.
<TextBlock Height="23" Width=20 HorizontalAlignment="Left" Name="textBlock1" Text="" VerticalAlignment="Top" > β </TextBlock>
Therefore, you should be able to use this text block as part of the template for the control.
I'm not sure exactly what you need, but I think this will give a better effect for using the image for the back arrow instead of using text.
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