The Several Bathrooms of O'Sensei tour of Japan. I'm seeing a lot of possibilities for new products here.
Perhaps these were the secret dojo(s) of O'Sensei where the art was really transmitted.
Imagine holding the soap for sensei, you need to protect the soap, as other uchi deshi will want to be the one that get to hold it for sensei, thus you must practice aiki against katatedori attacks and even multiple attacks, yet maintain proper posture or else you will fall over on the slippery floor.
FWIW Having recently been handed the secret recipe for the soap he used, I am now developing the proper handgrip to hold it, not too tight, not to loose...just using Ki. I am hopeful of learning the secrets of towel flicking in the future to develop ma-ai, generation of power etc... as well