In my browsing around the USDA's FSIS site I've seen pages that indicate there's a "Zero Tolerance" policy for beef feces. On [link|http://www.fsis.usda.gov/OPPDE/rdad/FSISDir7000.htm|this] page of directives for processed products I haven't been able to find anything about permissible levels of rat hair or feces. They have several directives about controlling bacteria, etc.
Are you sure this isn't an urban legend from the days before the USDA when hot dogs were made of sawdust and the like? I'd be very interested in a link if you can find one. Thanks.
Cheers,
Scott.