Salting the ice will help you with the last few degrees, it will lower the freezing point of the ice, but it can't touch the amount of heat that a given amount of ice can absorb (laws of thermodynamics and everything.)
And I think Bacchus answered whether anyone has had success with using the bathtub for chilling 5 gallons of wort.
If you had access to piles of ammonium chloride (pulling that from memory as a salt that has an endothermic reaction with water) it would work for chilling the wort reasonably fast, but long term it's going to be a heckuva lot more expensive than a copper wort chiller.
You really might want to research no-chill though. The Aussies have (apparently) had a lot of luck with it, and I've never heard of anyone trying it and then going back to the git-er-done chilling camp. (I've never done no-chill myself, but then I'm from the deep south, so git-er-done is pretty deeply ingrained in me.) From everything I've heard the only effective difference is that you pitch your yeast the next day. There's no noticeable DMSO off flavor or long term flavor instability, just half an hour less work on brew day and an extra 10 minutes on the day after.