...
 public UIElement InnerContent
    {
        get { return (UIElement)GetValue(InnerContentProperty); }
        set { SetValue(InnerContentProperty, value); }
    }

    public static readonly DependencyProperty InnerContentProperty =
        DependencyProperty.Register("InnerContent", typeof(UIElement), typeof(MyCustomControl));
...

<ContentPresenter Height="0"
                  x:Name="ToggleRegion"
                  Content="{Binding Path=InnerContent, RelativeSource={RelativeSource AncestorType=local:MyCustomControl}}">
</ContentPresenter>