Wouldn't the normal run of the mill stress track handling be up to doing the same thing as you're trying for with the plot armor? It's generally considered bad form to outright kill your PCs in any case. Especially if most of their power is tucked away in another form at the time. Even in the novels, Kincaid only uses the whole "I could snipe you from a mile away and you'd be dead before you even knew I was mad at you" as a social attack to keep Harry on his toes.
Technically the plot armor power could do something for a character but I think you'd either end up with a case where it does nothing ("you don't die" powers only come up when you would otherwise die... that doesn't happen often, one would hope?) or it's indispensable because people who don't have it are dropping like flies. Pricing it at -3 refresh seems to put it closer to the second category than the first.