I'm thinking all you'll need is a voltage comparator, a mosfet, some resistors and a reference voltage (a zener diode may be fine for this). The idea is that once your supply voltage goes lower than ~4.8v, the comparator will turn off the mosfet, which will act as a switch on the power supply line. Using a feedback resistor, we can also introduce some historesis so the power will not come back on for the rpi until the supply voltage is upto around 4.9v. I'm assuming your planning to get this done with surface mount parts, would this be correct? Also, depending on a number of different factors, we may be able to get away with using a zener diode, otherwise we may have to look at a more accurate voltage reference.