Can one supply an Arduino Zero's VIN pin with 5v?
Yes you can. If you look at the schematics on the Arduino website you can see that the 3.3 volt regulator is connected to the 5 volt regulator. That means that if you feed 5 volts straight into the 5 volt pin it will go through the 3.3 volt regulator to supply the main chip.
Note that this is going into the 5 volt pin not the VIN pin which goes into the 5 volt regulator and would thus require a higher voltage than 5 volts.