Adding a temporary load to a power supply
You could start by monitoring the load current like this: -



The output voltage (1 volt per amp of load current) could be used to detect that less than 20 mA is flowing into the load and turn on a transistor that shunts to ground whatever current is needed to make 20 mA flow through the 0.1 ohm shunt resistor.

If the load takes more than 20 mA then the transistor turns off.