If the system is designed so that, say, security at the casino can fire at bad guys without hitting other guards or casino staff (or so that the bad guys can't steal a guard's gun, and then go trigger happy on the other guards), then using a hostage is fair game. As for the rules, they'd be under the subdual rules, I reckon, but I must mention this is only working with the Safe Target System. And it doesn't mention that the safe target system can be easily turned off, meaning unless someone hacks the gun, like in the fiction in Twilight Horizon, you're in trouble.