I'd be very reluctant to do that, especially with a ringed engine where it could squash the top of the piston and jam the ring in the groove (or worse if it has a Dykes ring). At the best I'd invert the engine and fill with oil part way to get a hydraulic lock but even then I wouldn't use too much force. My way would be to take off the backplate and use a piece of wood to jam the crankshaft (either the crank pin or balancing cut out) against the crankcase side. Once the nut is off then find out why it was so tight.