Hi!
A possible solution not involving OpenStudio, but just excel:
Create the Pivot-Table in a seperate Excel file (call it pivot.xls), referencing the data from your output excel file (output.xls).
In theory, each time you open pivot.xls, Excel will ask you if you want to update the data from the external output.xml file. Even if excel does not ask you, you should be able to simply right-click on the pivot table and choose to update the data.
As long as output.xls is always stored to the same location this should work. When creating the pivot table you can choose to also "embed" the data, so that pivot.xls can be used on another system that does not have output.xls.
Hope that helps.
Regards from Vienna,
Richard Unger