Depending on which car model and year, the HV battery through the DC-DC inverter provides up to 200-250 amps at 12v (actually 13-16 v depending on the model). The 12v power outlet is limited to about 12.5 amps continuously. Currently, when the car is on, the DC-DC inverter is powering the car. It provides power to the power outlet. It has plenty of power to also charge the 12v battery if needed and run everything else. Keep in mind, when the car is on, you're going to lose about 1 mile per hour of range. This is because even if you are pulling 150W from the power outlet, there are other loads such as the CPUs, displays, etc. that are pulling power too.
As for warranty, I don't see how taking 150W from the power outlet would have any effect. My understanding of the statement "Do not use the Battery as a stationary power source. Doing so voids the warranty." has to do with using the car to power something major by bypassing the power plug. For example, hard wiring a 1500W inverter (125 amps) to the 12v system would clearly violate the warranty. Connecting a high-voltage inverter directly to the HV pack would also void the warranty. Now once your car is out of warranty, anything goes!