I've done this before for a customer. The trick we used was to have a
BarCode FONT. Then it was just a matter of placing text in the field.
If you need the barcode to be dynamic, then you will need to use InfoPath
2007, which does allow you to place a field value in the header/footer.
The other option, if you want it in the header, is to make your top-margin
as small as needed to have the first line or two look like a header.
However, this doesn't repeat across pages -- which is fine for some
applications.