I have Mac OS X 10.5.7 and Cups + Gutenprint v5.2.4. I can select the tabloid page size as the default or within the application. When the job prints, it does so on US letter size. In "paper handling" the destination paper size is suggest as US letter and cannot be changed. If 'scale to fit" is selected, tabloid is not an option. I have attached screen shot. The application is excel 2007.
2009-11-05 22:26:26 UTC in Gutenprint - Top Quality Printer Drivers