How to override a global style (that doesn’t have an x:Key), or alternatively apply a named style to all type-targeted controls?

Try this:

<Style TargetType="{x:Type StackPanel}" BasedOn="{StaticResource {x:Type StackPanel}}">
  <!-- ... -->
</Style>

I have declared my base styles in a ResourceDictionary in App.xaml, if i override them in a specific window like this, it usually works.