I've never seen it written or said that you can't use asymmetric reverse thrust. The only guidance I have seen is that it may "make directional control difficult", but if you're attempting to use it to re-gain directional control, that could be a bad idea (thrust changes don't happen immediately), unless loss of control in attempting to stop is preferable to not trying...
The reason reverse thrusters get locked out is to prevent inadvertent deployment in flight.