Take a small chunk of 2x4 of a size to fit between a jaw of the chuck and the rear way, such that it will prevent the chuck from turning. Put the lathe in the absolute lowest reverse gear. Allow a couple of inches between the chuck jaw and the piece of wood that will keep it from turning. Bump the starter switch for 1 second, so that the chuck starts to turn but hits the wood and is prevented from turning further. Since it can no longer turn with the spindle, it will begin to unscrew.
Since you only had the motor on for a moment at at a low speed it will not continue to unscrew, coming off the spindle, clobbering the ways and then landing on your foot. You can now unscrew it the rest of the way by hand, with appropriate protection (a board) on the ways.