Yes, it is currently not possible to perform a (predefined)command in
digi_ned on a hardware input.
The only way I know is a hardware reset through a reset of the PC. I once
performed a reset on PC hardware using e ZVEI 5-tone receiver.
One other posibility is to perfom such a action in Linux. It might be
possible to end a deamon or process on a hardware interupt. Something
similar is done with UPS where at low power the PC is shutdown
The last possible thing is to create the required code and add it to
DIGI_NED. All source is available under GNU.
Sorry I cannot be of help.