Create an aspx page that either shows your message or redirects to the Default.aspx of reporting services depending on the time of day. Set that as your default page in IIS on the Report Server and remove the Default.aspx from the default file types.
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Personal Motto: Why push the envelope when you can just open it?
If you follow the direction given HERE[/url] you'll likely increase the number and quality of responses you get to your question.
Jason L. Selburg