Is it possible to use a Zener diode as a High Pass Voltage
You you can use a ultra low power voltage monitoring IC. This IC or similar IC will keep the MCU at reset state even down till 0.7 V of \$V_{IN}\$ .    

   

As soon as the Voltage drops below the internal reference, the MCU will be held under reset. This will make sure that MCU will work normally when the input voltage is back in its valid range.