Pull-up or pull-down at the optocoupler output
As you can see in the datasheet, this optocoupler uses a (Schottky-clamped) transistor between the output and ground pins:



This output is designed to be used with a pull-up resistor.

The chip requires that its supply voltage (VCC relative to GND) stays constant, and the output transistor is switched on by raising its base above GND, so it is unlikely that you could manage to do anything interesting with the GND pin.