Is there any way to turn off the "loading" message when a page refreshes in Reporting Services?
If you have an ASP.NET developer on hand, have them use the ReportViewer control to embed the report directly into a plain old aspx page. I've done this to a few reports, and they never show the usual "Report is being generated" message that you get when accessing them via Report Manager. (You might have to set AsyncRendering="False" on the ReportViewer control to get the desired behavior.)

This will also allow you to dress up the page a bit, and conceal the various Reporting Services toolbars, which would be ideal for a billboard display like you describe. Put a meta refresh in the page to get a 30-second refresh, and you're set.