To create a custom document property, go to Word > Info > Properties > Advanced Properties > Custom. For a recent implementation, see:
https://www.msofficeforums.com/word-...numbering.html
As for updating a content control in a document header, you'd use something like:
ActiveDocument.Sections(1).Headers(wdHeaderFooterP rimary).Range.ContentControls(1).Range.Text = "Hello"