I THINK that if the jumper box is only applied to the starter side of the solenoid, in other words effectively straight to the starter terminal, and the key is in the on position and the starter button not pressed, it will work and the 12v part is not shared with the rest of the car.
I also THINK (correct me if wrong) that in the above scenario, you could even connect the jumper the wrong way around and it would not make any difference. Not that you would do that on purpose but you might do it by mistake.
Again, it is a jump straight to the starter and not connected to the rest of the vehicle in any way.
The above scenario (connected the correct way around) is not ideal, though, because it still requires the vehicle's own battery to have enough juice to spark the ignition. Having said that, even if a bit low, at least it is not being dragged lower by the starter being used in the conventional way.
Mart.