How to update a Setter within a MarkupExtension

I'm trying to implement my own MarkupExtension that takes two resource keys (in the SystemColor namespace) and a scale which mixes the two brushes. What I'm finding very difficult to do is implementing an update in case the brushes change. It works on a simple DependencyObject, but not on a Setter when used within a Style for a ListBoxItem.

I'm using the base code provided at: https://xamlmarkupextensions.codeplex.com/ NestedMarkupExtension, that provides the method UpdateNewValue(). I watch for the System event Microsoft.Win32.SystemEvents.UserPreferencesChanged. When I update the Setter with the MarkupExtension inside the setter, I get an exception that the PropertyInfo object is "sealed". The object isn't of type Setter though.

e.g. 

   <Style TargetType="{x:Type ListBoxItem}">
    <Style.Triggers>
      <Trigger Property="IsMouseOver" Value="True">
        <Setter Property="Background" Value="{c:DShadedColorBrush {x:Static SystemColors.WindowBrushKey}, {x:Static SystemColors.HighlightBrushKey}, 0.75}"/>
      </Trigger>
    </Style.Triggers>
  </Style>

If I use DynamicResource as such:

  <SolidColorBrush x:Key="MyBrush" Color="{c:DShadedColor {x:Static SystemColors.WindowBrushKey}, {x:Static SystemColors.HighlightBrushKey}, 0.75}"/>
  
  <Style TargetType="{x:Type ListBoxItem}">
    <Style.Triggers>
      <Trigger Property="IsMouseOver" Value="True">
        <Setter Property="Background" Value="{DynamicResource MyBrush}"/>
      </Trigger>
    </Style.Triggers>
  </Style>

Then I get an error that the DependencyObject id readonly.

There appears to be no easy mechanism, reading through the source for DynamicResourceExtension, everything is internal and there's no reuse I can use here.

To me, this seems an extremely useful thing to do, instead I've had to currently revert to Code-Behind to manually update the resource. Unfortunately, that's long-winded and not very reusable.