I had a go at upgrading the serializer to XSSF but there were simply too many Java class changes for lazy old me. Instead, I had much better mileage going down the OpenXML road and generating an .xlsx file in the Cocoon pipeline (it is only a zipped up bunch of XML files after all).
All the files are fairly straightforward XML and it is only the packaging that would be tricky but for the invaluable Zip serializer.
Inevitably, there is a lot of googling required to find examples but I recommend
as a good place to start.