So before I left, I was having a deep and abiding problem getting Microsoft's Open XML toolkit to work with a MemoryStream, because the MemoryStream couldn't be resized, nor could I find a way to get the resulting changes to the document written out to a disk file. This was after reading every bit of documentation I could find.
Today, it seems that my Google-fu turned back on, because I found the sample code that:
*thud* *thud* *thud*
This should not have been this hard to find.