How to calculate the power rating of zener diode for flyback protection?
You don't mention the inductance of your load, but if it's not a drive motor for an aircraft carrier it probably won't store enough power to heat up your zener.  Most likely, since you're driving it push-pull, the body diodes will provide flyback protection on their own, but the zener won't hurt anything.  In any case, current from your 3.3-27V supply will dump into your 24V supply through the body diode before the Zener starts to conduct significantly.  If you don't want to overtax the body diodes, you can put Schottky diodes across them.