Eric ... I have to admit, I hadn't ever thought of adding salt to the black water tank ... and am not a chemist, so I don't know how much you'd have to add to get a mixture that would alter the freezing point of water.
Salt water, however, does freeze at some temperature/combination/concentration
Reason I use RV Antifreeze to "lube" the tanks when we're winter camping is that RV antifreeze will also "freeze" into a slush but doesn't "expand" or freeze hard.
I'd be worried that your salt/water mixture would freeze, expand and possibly damage something.
Like Suz says ... the RV antifreeze is cheap.
If you are fulltiming
or doing some really extended winter camping, you could always add some tank heater pads or some automatic thermostatically controlled heat tape to your dump pipes and valves.
Don't make the mistake of using regular heat tape (the kind used on roofs to prevent snow dams) ... it gets too hot and could damage the pipes.