I agree with your solution. Since this is the ONLY place in the volume where the Nemeth notation is needed, place your transcriber's note before the table as you gave it. Then use the beginning and end Nemeh indicators right there in the table where they are needed. There are only two columns, so I think it will fit in the second column. And not to be picky, I think there IS provision for subscript notation. The provision is to use Nemeth for it. And they even tell you how.