Safety of Sous-Vide for long cooking times Note: all of the questions I saw on this focused on higher temperatures.

Brisket, being a fairly large and tough cut of meat, takes longer to cook than a a steak or many roasts. Would it be safe to cook a brisket sous-vide at medium-rare (131˚ F) or rare levels of doneness ?
The reduction or elimination of pathogens in cooked food is not simply a factor of temperature, but a factor of temperature AND time. Lower temps typically mean longer cooks.  This changes the texture of the final product.  Sometimes to the point that people find it unpleasant to eat. You will need to factor in the thickness of the cut, and the desired effect you are looking for, in order to determine the length of your cook.  Your best bet is to look here for a comprehensive treatment of the subject.  In fact, Baldwin has a short section on brisket.