Projection では裏を向いているときに非表示にするということができないみたいです。
なので自前で、
表を向いているときは Visibility を Visible に
裏を向いているときは Visibility を Collapsed に
としてやればいいんじゃないかと思います。
サンプルも探せばいろいろあると思いますが、とりあえず、単純に Y 軸回転でクルクル回る例を以下に貼り付けておきます。
<Grid x:Name="LayoutRoot">
<Grid.Resources>
<Storyboard x:Name="storyboard1" RepeatBehavior="Forever">
<DoubleAnimationUsingKeyFrames Storyboard.TargetName="frontPanelProj" Storyboard.TargetProperty="RotationY">
<LinearDoubleKeyFrame KeyTime="00:00:00.0" Value="0" />
<LinearDoubleKeyFrame KeyTime="00:00:00.5" Value="-90" />
<LinearDoubleKeyFrame KeyTime="00:00:01.0" Value="-180" />
<LinearDoubleKeyFrame KeyTime="00:00:01.5" Value="-270" />
<LinearDoubleKeyFrame KeyTime="00:00:02.0" Value="-360" />
</DoubleAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="frontPanel" Storyboard.TargetProperty="Visibility">
<DiscreteObjectKeyFrame KeyTime="00:00:00.0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
<DiscreteObjectKeyFrame KeyTime="00:00:00.5">
<DiscreteObjectKeyFrame.Value>
<Visibility>Collapsed</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
<DiscreteObjectKeyFrame KeyTime="00:00:01.5">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
<DoubleAnimationUsingKeyFrames Storyboard.TargetName="backPanelProj" Storyboard.TargetProperty="RotationY">
<LinearDoubleKeyFrame KeyTime="00:00:00.0" Value="-180" />
<LinearDoubleKeyFrame KeyTime="00:00:00.5" Value="-270" />
<LinearDoubleKeyFrame KeyTime="00:00:01.0" Value="-360" />
<LinearDoubleKeyFrame KeyTime="00:00:01.0" Value="0" />
<LinearDoubleKeyFrame KeyTime="00:00:01.5" Value="-90" />
<LinearDoubleKeyFrame KeyTime="00:00:02.0" Value="-180" />
</DoubleAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="backPanel" Storyboard.TargetProperty="Visibility">
<DiscreteObjectKeyFrame KeyTime="00:00:00.0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Collapsed</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
<DiscreteObjectKeyFrame KeyTime="00:00:00.5">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
<DiscreteObjectKeyFrame KeyTime="00:00:01.5">
<DiscreteObjectKeyFrame.Value>
<Visibility>Collapsed</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</Grid.Resources>
<Border x:Name="frontPanel" Width="200" Height="200" BorderThickness="1" BorderBrush="Black" HorizontalAlignment="Center" VerticalAlignment="Center">
<Border.Projection>
<PlaneProjection x:Name="frontPanelProj"/>
</Border.Projection>
<TextBlock HorizontalAlignment="Center" VerticalAlignment="Center">おもて</TextBlock>
</Border>
<Border x:Name="backPanel" Width="200" Height="200" BorderThickness="1" BorderBrush="Black" HorizontalAlignment="Center" VerticalAlignment="Center">
<Border.Projection>
<PlaneProjection x:Name="backPanelProj"/>
</Border.Projection>
<TextBlock HorizontalAlignment="Center" VerticalAlignment="Center">うら</TextBlock>
</Border>
</Grid>
青柳 臣一 (Shinichi Aoyagi)