I'll try not to stick my finger in the high voltage bits
So, how about if the car were stopped, turned off, and the booster battery was connected through the two high voltage cables used for the ChaDeMo connector, and the booster battery pack never exceeded 393.5 volts, can you foresee a problem with that?
Obviously, a three way switch to the battery, selecting ChaDeMo or booster battery. Now, with that simple addition, I can't imagine that the car would think anything but that it's being charged.
Having said that, obviously it's to our advantage to know the protocols and safety features of the ChaDeMo that may inhibit or regulate charging.