How do I access an element of a parent template style in XAML?

I am having trouble accessing a named element of a parent template style in WPF.

I have a custom control for a Button (written per this StackOverflow question):

CustomButton.xaml:

<ResourceDictionary ...>
<Style x:Key="ButtonCustomStyle" TargetType="{x:Type local:ImageButton}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type local:ImageButton}">
                <Grid>
                    <Image Source="{TemplateBinding Image}" Stretch="Fill" x:Name="CurrentImage"/>
                </Grid>
                <ControlTemplate.Triggers>
                    <Trigger Property="IsMouseOver" Value="True">
                        <Setter TargetName="CurrentImage" Property="Source" Value="{Binding ImageHover, RelativeSource={RelativeSource TemplatedParent}}"/>
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

MainWindow.xaml:

<Window ...>
<Window.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="CustomButton.xaml"/>
        </ResourceDictionary.MergedDictionaries>
        <Style TargetType="{x:Type local:ImageButton}" BasedOn="{StaticResource ButtonCustomStyle}"/>
    </ResourceDictionary>
</Window.Resources>

<Grid>
    <local:ImageButton Image="\Images\image.png" ImageHover="\Images\image_hover.png" Width="20" Height="20"/>
</Grid>

ImageButton inherits Button, and Image and ImageHover are defined as dependency properties. This code works great and does what expected.

Now, I want to extend this template style for a single particular button, adding an additional Trigger for changing the image of the button. I am trying to do the following:

<Window...>
<Window.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="CustomButton.xaml"/>
        </ResourceDictionary.MergedDictionaries>
        <Style TargetType="{x:Type local:ImageButton}" BasedOn="{StaticResource ButtonCustomStyle}"/>
        <Style TargetType="{x:Type local:ImageButton}"
               BasedOn="{StaticResource ButtonCustomStyle}"
               x:Key="Disableable">
            <Style.Triggers>
                <Trigger Property="IsEnabled" Value="False">
                    <Setter TargetName="CurrentImage" Property="Source" Value="\Images\disabled.png"/>
                </Trigger>
            </Style.Triggers>
        </Style>
    </ResourceDictionary>
</Window.Resources>

<Grid>
    <local:ImageButton Image="\Images\image.png" ImageHover="\Images\image_hover.png" Width="20" Height="20"/>
    <local:ImageButton Style="{StaticResource Disableable}" Image="\Images\image.png" ImageHover="\Images\image_hover.png" Width="20" Height="20"/>
</Grid>

But Visual Studio underscores the new setter line and says 'The name "CurrentImage" is not recognized'.

Is there a way to alter the CurrentImage element?