Is there such as thing as an ideal Zener diode?
What you'd probably want is a "crowbar circuit", ie. a circuit that uses e.g. a MOSFET to short the supercap if voltage crosses a certain threshold.

An example of that can be found in the Ettus USRP N200 / N210 Schematic file on page 12:



You could implement that thresholding using a Zener diode with a significant series resistor. 

Another example of a crowbar overvoltage protection approach can be found on wikibooks.