What do you mean by driveline brakes? If you mean a rear output mounted e-brake, don't run one but it is legal and they work well.
If you mean pinion brakes, you can't run them on a normal axle. The rotor would be a few inches off of the ground sitting on level pavement.
Problems with both driveline e-brake and pinion brakes, if an axleshaft, driveshaft(not an issue with pinion brakes) or something else down the line fails, you have no braking. Also you have to deal with all the slop of the driveline. Any slop in the driveshaft, backlash of the ring and pinion, play in axleshafts, all that will effect it.
Eventually I plan to put a tcase e-brake on my S10, just not at the moment.
Oh and I know in the state of Pennsylvania these systems are legal. Parking brakes have to be a seperate system from service brakes, and there is no specification for mounting point of either service or parking brake, just a requirement of performance, which is easy to meet.