A colleague has an upcoming project where the robot carries a high-torque powered bolt driver. There are some concerns that the torque of the bolt driver may exceed the servo-holding limits of the robot. So they have been looking for ways to deliberately engage the robot servo brakes from inside a TP program.
The simplest solution, of course, would be to reduce the brake delay time to 0, but that would mean the brakes would perform a close/open cycle every time the robot hit a WAIT command.
The Fanuc System Variables contain $MCR.$BRK_OUT_ENB and $MCR.$BRK_OUT[1-8]. The documentation is very sparse, but lead us to believe this might allow the direct brake control we desire. However, experiments with our lab robots did not give the expected results -- turning on the _ENB causes a Warning message (JOG-012, "Manual Brake Enabled") that blocked the robot from moving. And even turning _ENB back to False did not clear the problem -- we had to force $MCR_GRP[1].$TURN_ON_SRV to True (it reset itself to False immediately) to make the robot movable again.
We did stumble across a method to make an axis go "limp", with neither brake holding or servo holding. Fortunately, we didn't break anything expensive. But that's not the effect we want anyway.