Yes, soap has sometimes been used to lubricate enema nozzles.
For example, the original JBL sit-on bag came with a box of several soap sticks. The instructions said to moisten a soap stick with water, insert it, and move it in and out somewhat to thoroughly lubricate the passage before sitting down on the nozzle.
It has pluses and minuses.
From the JBL book, The Royal Road:
“The reservoir having been filled as directed and the above directions carefully observed, the “Cascade” should be laid down and the “injection point” screwed in. It is then ready for use. Being all ready, the stick of rectal soap should be dipped in water—to moisten it—inserted in the rectum and withdrawn. This is simply to lubricate the passage and facilitate the admission of the “injection point.” Then, standing in front of the seat on which the “Cascade” is lying (as if preparing to sit down), pass the left hand between the lower limbs and grasp the handle of the faucet, to guide the “injection point” into the rectum, and then carefully sit down upon the “Cascade.” When the “injection point” has been completely introduced and you are comfortably seated, relax the muscles and allow the whole weight of the body to rest freely on the “Cascade,” and turn on the faucet, partially at first, then, after a few seconds, turn it on fully and you will readily receive the water.”