PS -
Each of the controls for "AccountName" are bounding boxes. I ensure they are unlocked:

Still no luck with anything in the Page Headers. I do have Initial Page Header set to none, so the page header resides in another section, if that makes a difference.

Sub CCLoaderHiMum()
Dim aCC As ContentControl, aRng As Range

For Each aRng In ActiveDocument.StoryRanges


For Each aCC In aRng.ContentControls

If aCC.Title = "AccountName" Then
aCC.LockContents = False
aCC.Range.Text = "hi mum"
aCC.Range.Font.ColorIndex = wdBlack
aCC.LockContents = True
End If

Next aCC

Next aRng
End Sub