Использование LayoutTransformer
из набора инструментов sl
<xmlns:layout="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Layout.Toolkit">
<toolkit:Expander.HeaderTemplate>
<DataTemplate >
<layout:LayoutTransformer>
<layout:LayoutTransformer.LayoutTransform>
<RotateTransform Angle="-90"/>
</layout:LayoutTransformer.LayoutTransform>
<TextBlock Text="Some Text" />
</layout:LayoutTransformer>
</DataTemplate>
</toolkit:Expander.HeaderTemplate>