The answer given by Bob Bao [MSFT] was to put the background image in a rectangle with IsHitTestVisible set to false.
basically
<Grid x:Name="LayoutRoot">
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<Rectangle IsHitTestVisible="False" Grid.RowSpan="2" Margin="0,22,0,0">
<Rectangle.Fill>
<ImageBrush TileMode="Tile" ImageSource="images/MyImage.JPG" Stretch="None" AlignmentX="Left" AlignmentY="Top" />
</Rectangle.Fill>
</Rectangle>
<ribbon:Ribbon x:Name="Ribbon">