Is very slow smoking of meat allowed commercially in the US?
There are several additional questions that your original question brings about, if you are in the context of the USDA and regulations.  Whether or not the USDA allows for this kind of sausage production is a different question to whether or not there are producers who are actually doing it.

Are you interested in producing the sausages yourself to sell, or are you just looking for people who might be producing them for you to buy?

The USDA website for its Food Safety and Inspection Service, who would likely regulate this kind of production, is here:  https://www.fsis.usda.gov/wps/portal/fsis/home

This would be where you would look to find out if the regulations prohibit the production, or if there are specific prohibitions to ingredients or processes that are part of the production.