Circuit to protect against undervoltage?
According to your description, something like this should work:



The TLV3012 has the reference voltage built-in, so that's 1 6-pin IC, 1 FET, and 2 resistors.  Not sure if you consider that "simple".  :)



Similar:



Using a pMOS to switch a load is summarized here.

You could probably use a Zener as the reference for the comparator, too, or maybe rig something up to drive the MOSFET from the Zener more directly, but I don't know how precise that would be.