How can I protect this 48VDC device from overvoltage?
This is a concept that would typically work, you'd have to run the sums to see if it will work in all possible scenarios. 







simulate this circuit &ndash; Schematic created using CircuitLab

Q1 (a DN2540) acts like a resistor of some tens of ohms under normal conditions, so it drops some hundreds of mV typically. If the input voltage rises to allow the 48V zener to conduct, the gate-source voltage decreases to -3.5V and Q1 is guaranteed to be fully off. If the tolerance on the 48V zener is 5% (50.4V max), then the output voltage should not exceed 53.9V (a bit close!). 

R2 and the 12V TVS are just to protect the gate (+/-20V abs max) against any transients on the power supply.