I've just had another think on this one. If you have flattened your battery, and that's all you've done, then properly bump starting the car should do no harm. The first time a piston hits TDC, the engine should start in exactly the same way as it would have done using it's own starter motor and battery.
However, if the car will not start on it's battery and you have cranked and cranked in hope until the battery is flat, then attempting to bump start it would not be a good idea. Ten to one it will still not start and you are pushing unburnt fuel through the cat. Then again, cranking a car that will not start also pushes unburnt fuel through the cat, so what's the difference?
Steve