A) Move this functionality to another separate page (or even a custom HTTP handler instead of a full web form page).
B) Create a "splash" page with your waiting message. This page will redirect to the new page/handler created in (A).
C) Instead of the postback button, use a regular link to the splash page (B).
The browser will sit on the splash page until the new content comes up. Getting rid of the splash will be a bit more tricky because the behavior of the client will vary by browser. In I.E. with MS Office installed Excel till be embedded right in the I.E. document window and the splash screen will be replaced by it. On Firefox, you'll get a prompt to download/open the file in the appropriate app.