Caliburn Micro with TreeView ContextMenu target does not exist - wpf

Question

It seems (from other posts with similar problems) that I am trying to bind the target to an element which doesn't exist in the same visual tree. I have a ContextMenu on the 3rd level in the TreeView, and am trying to set an action to execute a method on the ViewModel via the Tag of the parent element (TextBlock), but can't get it to work.
The XAML is:
<HierarchicalDataTemplate x:Key="FeatureTemplate">
<Grid HorizontalAlignment="Stretch">
<TextBlock Text="{Binding Name}" Tag="{Binding DataContext, RelativeSource={RelativeSource Self}}">
<telerik:RadContextMenu.ContextMenu >
<telerik:RadContextMenu cal:Action.TargetWithoutContext="{Binding Path=PlacementTarget.Tag, RelativeSource={RelativeSource Self}}"
cal:Message.Attach="[Event Opened] = [Action ContextMenuOpened($source)]">
<telerik:RadMenuItem Header="Export" cal:Message.Attach="[Event Click]=[Action ContextMenuClick($source)]"/>
</telerik:RadContextMenu>
</telerik:RadContextMenu.ContextMenu>
</TextBlock>
</Grid>
</HierarchicalDataTemplate>
<HierarchicalDataTemplate
x:Key="FormTemplate"
ItemTemplate="{StaticResource FeatureTemplate}"
ItemsSource="{Binding List}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Image Source="{Binding Image}" Width="32" Height="32" HorizontalAlignment="Right" Margin="0,3"/>
<TextBlock Grid.Column="1" Text="{Binding Name}" VerticalAlignment="Center" Margin="5"/>
</Grid>
</HierarchicalDataTemplate>
<HierarchicalDataTemplate
x:Key="JobTemplate"
ItemTemplate="{StaticResource FormTemplate}"
ItemsSource="{Binding List}">
<TextBlock Text="{Binding Name}"/>
</HierarchicalDataTemplate>
<telerik:RadTreeView Name="TreeView1"
Grid.Row="2"
cal:Message.Attach="[Event SelectionChanged] = [Action SetSelectedItem($this.SelectedItem)]"
Background="White"
ItemsSource="{Binding AvailableData}"
ItemTemplate="{StaticResource JobTemplate}"
telerik:AnimationManager.IsAnimationEnabled="False" >
</telerik:RadTreeView>
ViewModel code:
public void ContextMenuOpened(FrameworkElement source){}
I get the error that ContextMenuOpened target is not found.
Any help or suggestions would be much appreciated.