I just checked and the calculated field does work in a textbox in the header that appears to be in the body of the document.

To reserve room for the field in your table, you may want to use a separate cell in the header row (possibly without a dividing border) so that the space will remain blank in the header row.

As was stated, placement of this will be a pain. You just have to experiment until it looks right. If someone unaware of the use of the textbox attempts to edit the document, they are likely to have problems.