How to protect low-voltage hardwired device against 120 V AC miswiring ? (I need a resettable solution)
You could add a sizeable diode between GND a V+ with some sort of reset fuse.
This will short the input on the AC negative wave and trip the fuse fast.
The tricky part is to protect the electronics behind for 20ms at 120V (50Hz). A big 120V cap with a 100ohm serial resistor (if you can accept the voltage drop) and a resettable fuse might do it. Some LDO that can take high transients can help.
Some ideas:


Add discharge resistor on the cap.

It would need to be tested and the components adjusted depending on your circuit.