Paper keyboards (and pedalboards) can be really helpful. The most important aspect of such practice is of course mental. We have to imagine the sounds in our heads. Also this is a powerful device for increasing one's focus.
It's a great way to stay on top of your organ practice when you are traveling. I personally use it when I prepare for recitals and have to stay in the hotel room without a real instrument available.
Another possibility is to have a recording of your piece in a slow practice tempo which could be used while playing paper keyboards and pedalboards.
I have also a warning here: since the touch of the paper keyboards is completely different from either mechanical or electronic organ, once in a while it is necessary to have an access to the real organ for practice.
But even when practicing on the table and on the floor, always be aware that your hand and feet position, depressions and the releases of the imaginary keys and pedals are the same as on the real instrument.
If you would like to learn more about organ practice, check out my new Organ Practice Training. It's intended for organists who struggle with various issues of their daily organ practice.
By the way, do you want to learn my special powerful techniques which help me to master any piece of organ music up to 10 times faster? If so, download my video Organ Practice Guide.