If it's connected via HDMI you can actually do that. There's a package called ddccontrol. For example, here is my code to toggle my 4th monitor between hdmi and displayport (hooked up to desktop or hooked up to a client laptop - this also switches out my synergy config at the same time later in the toggle script).

  CURRENT=$(sudo ddccontrol -r 0x60 dev:/dev/i2c-3 | tail -1)

  if [[ $CURRENT == *"+/15/3"* ]]; then
    sudo ddccontrol -r 0x60 -w 17 dev:/dev/i2c-3
  else
    sudo ddccontrol -r 0x60 -w 15 dev:/dev/i2c-3
  fi

By passing other parameters you can control brightness, volume, power, etc.