If they decide the guy at the back indicates an odd number of black hats by saying white or an even number of black hats by saying black. Now the remaining prisoners all live and the back guy has a 50/50 shot.
Glad you like it. Heard this as an interview question at a trading company. Gave something similar to your solution and was asked for a better one, eventually work my way to the solution above but was given no indication if it was what they wanted. It wasn't a pleasant experience.