We currently do this with Webi now that you can point to an Excel file that lives on BI Platform.
We have the owner of the Excel file upload to the BI Platform. The Webi is setup to look at this Excel file on the platform.
As you noted, as long as the format stays the same, all you have to do is refresh the Webi to pull in the updated Excel data.
This was new for us as we were on 4.0SP4 and now 4.1SP4. In the old version Excel had to be local to the PC Webi Client tool to refresh.
james