You don't need to use a zener to obtain the flywheel functionality (assuming you only drive coil current in one direction). Any PN junction diode capable of handling your coil voltage as a reverse voltage, and the coil current (plus some margin) as its forward current, will do.
You could use a zener to simultaneously protect the drive circuit from flywheel currents, while protecting the relay from over-voltage. But you'd need to do something to limit current into the zener in overvoltage conditions and this would also affect the coil current during normal operation.
However, the coil is probably more robust to the kind of things zeners could protect it from than whatever is driving the coil, so I'd stick with just using a silicon PN diode for flywheel protection and not worry about a zener.