In CLDR 1.8.1 http://www.unicode.org/cldr/data/charts/supplemental/territory_language_information.html some additional territories have been accepted as Letter paper using regions through submissions from http://unicode.org/cldr/trac/ticket/2585 affected are es_SV, es_PA and es_NI. Some supplementary evidence of government pdf forms in Letter format in addition to the direct users comments of above can be found at http://wiki.services.openoffice.org/wiki/DefaultPaperSize#Probable_Best_Fallback_Mapping
Created attachment 4773 [details] like so
I applied the patch as far as needed.