Technically, the car will not have any problem doing what you want. 240 Watts (20A @ 12V) is nothing for the size of the battery in the e-Golf as long as it is in READY mode. While I don't know exactly the specs of the DC/DC converter, other EVs can sustain 100 amps @ 12V. It's just a matter of what measures VW put in place to prevent you from doing what you want. For example, if you drive the car any significant distance, then get out of the car, it will shut down in about 15 seconds. However, if you start the car and don't move it, it will stay on longer, even if you get out. I never bothered to see how long it will stay on.
I summoned some google-fu and found this
High voltage becomes vehicle power circuit voltage. As mentioned above, the 2.5-kW DC/DC converter integrated into the power electronics is responsible for supplying the vehicle’s 12-V power circuit and thus works like a transformer. The 12-V power circuit and the high-voltage circuit are completely separate from each other in the vehicle. Also included in the power electronics are the controller for running the management software and a CAN interface for communication with control devices. Last but not least, the power electronics module dampens the effects of any sudden loading of the drive system (for instance, at moments of sudden acceleration) by regulating the torque accordingly.
So... more like 200 amps!