I now send out low res images to have them keyworded - then I use GraphicConverter to copy all the metadata from the low res jpg files to the high res tiff files. Files have to be named identically, but can be different types.
Last weekend I was with a volume-microstocker that outsources his metadata and he used - weirdly enough - a Nikon program that does exactly that: automatically copy the IPTC from the lores jpgs to the production TIFs with the same file name. He's under Windows.
The spreadsheet method of Jazz sounds awkward at first glance, especially when uploading to many sites. Is there a way (desktop app) you can hard copy the spreadsheet data into the image IPTC?