I am using GS 8.13 to output PDFs to either PNG or JPG files, one file
per page. By default, the pages will come out using the rotation
specified by the PDF file. Is there any way that I can force all pages
to be of a certain orientation?

GSView does it, you can pick portarit/Landscape etc from the
orientation menu and it will fix all pages to that. I get the feeling
though, that this feture is GSView specific code and not something
that is build into GS?

Thanks.