I am negative :-(
This rule is evidently designed for security. If anything from doctors can be a permit to bathroom, then not only UC patients but others can get it, which means the rule does nothing... I have ever taken a 12-hour flight with moderate flare (6-8 times bathroom a day). What I did was to have more food the day before flight and less food on the day flight. This can reduce some 'solid' stool. I went to restroom whenever possible/ready at airport and on plane. Also, I put toilet paper in underwear. I saw some posts here saying nappy/sanitary towel is good as well. But I like toilet paper because it does not draw attention. Can you imagine a guy taking sanitary towel to restroom several times
Take 'enough' underwears and pants; make your package as small as possible so that you can run. Do not make your transition tight so as to change underwears if necessary. Last but not least, I often train/tell myself to not feel shy with wet pants no matter how others look at me...