Component selection criteria:
LM4040AIM3-2.5 has a minimum operating current of around 60uA.
LT1716CS has a quiescent current of around 35uA and will operate from a single-ended supply up to 44V. It also has a class B output so doesn't require a pullup resistor.
P-channel FET is not critical. I used something I had on hand, Vds and Rds(on) are the most important parameters.
Trigger level and hysteresis thresholds are adjustable. The scope waveforms show the switch turning off at 12.0V and turning on at 12.4V but these are completely arbitrary.
The PCB has been shared at OSH Park ($0.80 each).