Landscape (the opposite of portrait) means that the page is turned on its side. You can redefine showpage in terms of the current definition of showpage.
Do something like:
/oldshowpage /showpage load def
90 rotate llx neg ury neg translate % for the first page
/showpage
{
oldshowpage
90 rotate llx neg ury neg translate
} def
This won't work if the PostScript file you're editing uses initgraphics or grestoreall. Also note that the method described (redefining showpage) does not conform to the document structuring conventions. The Adobe recommended method involves performing the transformaton as part of the setup for each page.