There should be a default option to banish cards face-down, as well as reveal them while they are face-down in the banished zone.
There is such an option already, however it will not show up unless there are already cards banished face-down by the effect of cards such as pot of desires or eater of millions. I would just like to be able to test out custom cards that banish things face-down without having to apologize constantly because the program won't let me banish things face-down without having played specific cards first.