Jack,
Enable the A axis and assign pins to it. Set the velocity and acceleration to maximum values. Connect a solenoid control to the A axis direction pin. In your gcode, a G0 A-.0001 should trigger the solenoid. A G0 A0 should turn off the solenoid. If the solenoid works backwards to what you want, then reverse the Dir LowActive setting.
Do the same for the B and C axes.
Regards,
John Champlain