Whether it's a reasonable thing to do, I don't know, but one way you could do this is to put a switch on the machine, connected to the Mach3 PROBE input, to tell you when you've reached the end of the cut. Then do the actual cut with a G31 move. You'd have to start the spindle first, and reverse the spindle to move back to the starting point. As long as you only cut one direction, backlash should not matter. I suppose you could also do in in units/rev mode, and do away with the switch. The trick will be coming up with a foolproof means of determining the end of cut, before you run into the chuck. Or, just cut away from the headstock, and stop it manually after you run off the end of the part.
Regards,
Ray L.