If you use a checkbox content control titled 'Chk1', for example, and a corresponding text content control titled 'Chk1Inits', you could use a ContentControlOnExit macro like:
Code:
Private Sub Document_ContentControlOnExit(ByVal CCtrl As ContentControl, Cancel As Boolean)
Application.ScreenUpdating = False
With CCtrl
If .Type = wdContentControlCheckBox Then
With ActiveDocument.SelectContentControlsByTitle(.Title & "Inits")(1)
.LockContents = False
.Range.Text = Application.UserInitials
.LockContents = True
End With
End If
End With
Application.ScreenUpdating = True
End Sub