I suspect this can't be done via ASP.NET, as the orientation is set typically as one of the printer properties. You have no access to the printer on the client. Also, you are not actually dealing with an Excel file. You are creating an HTML file that Excel is happy to recognise up to a point.
You may be able to achieve what you want by creating an Excel document as a kind of template and putting it on the server, then using the Jet OleDb provider to write the values to it.