This is not a normal use or operation of a car. Thus you will not win in court should a warranty claim be denied. There are a lot of things the manual doesn't explicitly say you shouldn't do. Driving off a cliff for example. Using the car as a snow plow. Even with 800 pages in the manual, there simply isn't enough room to cover all the things the car was not designed to do.I appreciate your concern, but there is nothing in the warranty that says you can't temporarily connect an external device to the car battery. It's the same as connecting jumper cables and starting a friend's car. Every circuit is fused and protected. The only thing the manual warns against is replacing a fuse with a larger one, which would void the warranty.
While more power to you if you make it work with no downside, I can only imagine your 12 volt battery will be killed in short order with a longer test. It simply isn't made for it, nor was the DC/DC converter. Far better to connect an inverter directly to the traction battery and take out the stuff in the middle of your circuit. I believe that is how Prius owners hook up to house power.