It turns out that the Office app (hub) doesn't support reading from SD cards. :(
I then wondered if it was possible programatically.
A quick check of the file extensions that are reserved and not available to 3rd party apps did not include any of the ones I was interested in: .doc; .docx; .xls; & .xlsx.
It seems that MSDN wasn't telling the whole truth. There is no 3rd party access to file with those extensions either. :(