Thanks for your feedback. We will consider your suggestion for the future versions of the Window. As for now I can suggest you another solution which can be done in the code instead of the modifying the ControlTemplate. You could get the element named HeaderContent from the Template and change its HorizontalAlignment property to Center the following way:
var radWindow = new RadWindow();
radWindow.Header = "My RadWindow";
// change the alignment
var headerContent = radWindow.Template.FindName("HeaderContent", radWindow) as ContentPresenter;
headerContent.HorizontalAlignment = HorizontalAlignment.Center;
Hope this will work for you.
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