Number 1 is horse manure. I flew an in pressurized T-37 for 3-yrs and up to 25,000 feet. No problems, and never heard of such a thing fleet-wide or in the history of the aircraft - altitude chamber training mentioned gas expansion and never once mentioned this “problem.”
So I guess you really mean UN-pressurized...
I'm sure it would be uncomfortable, but unless there is a blockage of the intestinal tract, I can't see a rupture occurring.
And, if there was a blockage, they were going to have a problem anyway.
One thing Brits found was that large number of bombs were
hung up in racks
Was traced to crew members peeing in bomb bay as plane made its climb
Pee would freeze at high altitude cold and cause bombs to hang up in racks.....