I just tried it on my M9712.
Connecting a 24V supply, current limited to 250mA, in reverse to the M9712 does short the supply out but does not make any buzzer sound, nor does it show REVERSED in the display....
If the load is ON there's basically a dead short (MOSFETs conduting in reverse while "on"), display show UNREG, no buzzer.
If the load is OFF there's the ~0.6V drop across the intrinsic diode(s), display shows OFF, no buzzer.
Can anyone else verify?