xlbook - Open through the Excel Add in?

This is currently not possible, sadly. If you look at the File Explorer in the Client, when you right-click on a book, it says "Process and Open" - the server has to do some work on the book before it can be pushed to Excel. Even from the built-in Spreadsheet page, if you try to Open -> From XF FileSystem, you cannot select a book (notice they are greyed out).

You can contact Support and ask them to file it as a feature request. Sorry!