If you think it through, the point of the ducolax for people with a colon, is to be sure you are not constipated (full of stool) in the rectum at the time of the MRI. If you usually have rather loose stools (typical for a j-pouch), then a laxative won't affect anything one way or the other.
I would call back for clarification, pointing out that you do not have a colon and do not have formed stool. Be sure to have them ask the radiologist, not just read from the instructions. But, I think you can skip it. If you really need an empty pouch, then an enema before leaving the house would work better. I have not had a pelvic MRI, but have had oodles of pelvic CTs, and it has not been an issue. I do realize that the location they are looking at makes a difference.
Jan