Property similar to Label.Target for a custom templated control - wpf

Question

I have a control which template contains a generic ContentPresenter. This ContentPresenter will normally contain an AccessText (possibly amongst other things), whose AccessKey should focus the whole control. How do I do that?
Here is a specific but much simplified example to my case. (I know it could be implemented in several other ways (e.g. UserControl), but I need it this way for various reasons). I just made it up, there are minor omissions; the point is only to show the idea.
Imagine I want to add a header to the TextBox. I subclass it into a TextBoxEx and add two dependency properties Header and HeaderTemplate, similarly to HeaderedContentControl. (Code is trivial and omitted).
Then I assign a template to my TextBoxEx along these lines:
<ControlTemplate TargetType="TextBoxEx">
<StackPanel>
<ContentPresenter Name="PART_Header"
Content="{TemplateBinding Header}"
ContentTemplate="{TemplateBinding HeaderTemplate}"
RecognizesAccessKey="True" />
<TextBox ... /> <!--or some other components with similar functionality-->
</StackPanel>
</ControlTemplate>
When using this control, I might do something like this:
<TextBoxEx Template="{StaticResource TemplateAbove}" Header="_Test">
<TextBoxEx.HeaderTemplate>
<DataTemplate>
<AccessText Text="{Binding}" FontSize="14" />
</DataTemplate>
</TextBoxEx.HeaderTemplate>
</TextBoxEx>
(Of course, in reality it is done via styles and is more elaborate, but I tried to conflate it to a minimal code).
Now, this all renders fine. The only problem is that when I press Alt+T, nothing happens. (Except for underlining the T). What I want is for the control to get focus - which is the usual action OnAccessKey.
Typicaly, such things are done using a Label, which has the Target property, which can point to the control to be focused. But here at the ControlTemplate level, I don't know what header template the user will use; it may not have anything AccessText-related at all, and I don't want to restrict it in any way. Yet if such control is used, I want the access key to work. Supposedly, ContentPresenter.RecognizesAccessKey should help, but how do I tell it what exactly to do, i.e. what (sub-)control to focus?
Or is there another easier approach (but still, on a template level)?
P.S. The problem can be solved by replacing the ContentPresenter with Label as follows:
<Label Name="PART_Header"
Content="{TemplateBinding Header}"
ContentTemplate="{TemplateBinding HeaderTemplate}"
Target="{Binding RelativeSource={RelativeSource TemplatedParent}} />
But I feel this is not the 'right' way to do it and I'm still missing something fundamental, so I'll happily listen to any suggestions.