Could an OB axis be used for this? Just set it to jog and then run a gcode?
SwiftyJ is on a roll! It could be possible, but not simple or easy, if all of the requirements line up. First, your motion controller will have to support the use of out of band axes. Not all do. Then you would have to use the motor mapping API calls and unmap the motor from the A axis and map it to an out of band (OB) axis. But you would have to set the feed rate for the OB axis with the API, meaning you can't set it with the F word in G code. And you will have to manually set the machine up in the screen load script so that the machine starts up with a known motor configuration, as changing the motors on the fly will play havoc at shutdown or if some external plugin or script saves the Mach settings.
Here are the API calls that would need to be used, at a minimum. (remember to check your API function return codes)
mc.mcAxisUnmapMotors(inst, axis)
mc.mcAxisMapMotor(inst, axis, motor)
mc.mcMotionSync(inst) -- to be used when the motor is mapped BACK to the A axis.
mc.mcJogSetFeedRate(inst, axis, unitsPerMin)
mc.mcJogVelocityStart(inst, axis, dir)
mc.mcJogVelocityStop(inst, axis)
Steve