PX4: disable PWM mixer

I am developing a feedback controller for a quadrotor that sends signals (the target angular velocity or voltage) directly to the motors. All the code examples I saw do something like

void Run() override
{
	// ...
	actuator_controls_s actuators;
	actuators.control[0] = ktx * torque_x; // forward-direction torque
	actuators.control[1] = kty * torque_y; // right-direction torque
	actuators.control[2] = ktz * torque_z; // up-direction torque
	actuators.control[3] = kfz * force_z; // up-direction force
	actuators.timestamp = hrt_absolute_time();
	orb_publish_auto(ORB_ID(actuator_controls_0),
		&_actuator0_pub, &actuators, nullptr, ORB_PRIO_DEFAULT);
}

So, all the signals actuators.control go throw the MultirotorMixer to pwm. But I would like to remove all the mixers from the pipeline to be able to control the motors individually.
As far as I understand, this can be made by changing the Multirotor mixer by the Simple mixer in quad_x.main.mix. So, I did

M: 1
S: 1 4      0  20000 -10000 -10000  10000

M: 1
S: 1 5      0  20000 -10000 -10000  10000

M: 1
S: 1 6      0  20000 -10000 -10000  10000

M: 1
S: 1 7      0  20000 -10000 -10000  10000

and also in C++ changed the channels to 4…7 of group actuator_controls_1.
This trick doesn’t work, because PX4IO::task_main() subscribes to the group actuator_controls_0, and does not react on messages in actuator_controls_1.
All of these seem to me like a dirty hack. Is there a good way? I will be surprised if not.