If the measurement is taken aft the driveline, it is still considered "rear wheel" or "Chasis dyno" numbers. Flywheel dynos are mated directly to the flywheel with the engine out of the car and a viscous coupler is what puts the load on the engine.
T56 driveline loss is about 13%, give or take. This is a very subjective number.
That setup is still a rwhp rating since it does go through the drive line and there will be the usual parasitic losses associated. The setup helps eliminate the potential of the tires being a source of inconsistancies such as slipping due to traction, air pressure, how well the car is tied down etc... The claim is that it is a more accurate method.