In most cases a normally closed valve would have to be fitted instead of the normally open type usually used. I think to meet the law you would have to have both front and rear systems actuated together or else you could face a locked rear braking system at highway speed if it fails.
Air brake systems on semi's are similar I believe (correct me if I am wrong) and they are legal. There are pinion type disc brakes that can function as emergency brakes, I think this would be a better bet for a rod.
Why do you want to do this? Is there a reason why your mechanical system has been disabled/removed?
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”
— Han Solo