You'd want them in a sheltered spot and on a good high platform where you could be REALLY sure that snow or mud or wet shavings will not build up around 'em, because the plug (where you plug in your extension cord) is right along the edge of the bottom of the waterer. Also, as much as they already require a GFCI-protected outlet for safety indoors, they SURE would outdoors.
But as long as you could get around the electrical safety challenges I don't see why not. The 3(?) gal plastic waterer with the integral heating element in its base, that a lot of places are selling, claims to be good down to 0 F.
JMO,
Pat