Sam showed me how to force a linux box to reboot, even if shutdown gets wedged on a stuck device. In our case, we had a devicemanager device that had hung (which you could see by running iostat -xm 1)

We use softdog, the nonhardware assisted watchdog driver to hard reset the machine if needed. softdog will hard reset the machine if it doesn't receive data every 60 seconds, so we open /dev/watchdog for write and then issue a shutdown -r:

modprobe softdog
sudo bash -c "cat > /dev/watchdog" &
shutdown -r now

Thanks, Sam!