You can disable mouse and keyboard input using `xinput --list` to list the input devices by name and id, then using `xinput --disable device-or-id` for each of the real mouse and keyboard devices. To reverse use `--enable`. 

Alternatively, you can use `sudo evtest --grab /dev/input/eventN &` where N is the number of the input device. The grab lasts until the process is interrupted.

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Another thing to try is setting the screen to stay on in the Xserver, then use `ddccontrol` to talk directly to the monitor over the DDC I2C bus. If the monitor is recognised, you can give a command to turn it off. Presumably, the Xserver will not try to turn it on if it assumes it is still on.