CEFOSKEY, the rotor retard idea would work since the machine would turn to keep the same airspeed for each rotor system. Transient performance would also be good as the rotor inertia was transferred to the fuselage during braking, until forces reached equilibrium. However, you would lose some power and generate heat - particularly in HOGE where you might not want to (pusher prop tail might force lots of peddal work in strong winds). Still in principle rotor retard works better than the tip brakes, since the rotor retard system would directly control yaw rate while the tip brakes only controlled yaw torque.
That does raise an interesting point about the differential collective in X2 though: Does the system only produce a good yaw rate control, in all flight regimes, because of the yaw laser gyro in the control system? Nick, has commented that S69 was not as positive in yaw as a tail rotor.