IMO, that's the sort of idea that was dreamed up by some fiction writer who thought the idea would add interest to their story. Seems I recall reading something like that in a story about the buffalo hunters.
Now, I can't say for certain but the buffalo hunters whole lives depended on their rifles so the idea of pissing on or in them simply doesn't make any sense. (The old expression of, "Piss on it" never is used in a complimentary way. It always is used to show total disrespect for something.)
All but the most ignorant people know urine contains a lot of salt in it and they also know that salt causes rust on steel or iron. In other words, salt water or urine is about the last thing anyone would want to use in their barrel.
That's my story and I'm sticking to it.
As for using boiling water to clean one of my muzzleloaders, I would never do it.
It's not that I'm so concerned with the barrel or the stock. It's I am concerned about damaging ME.
Just a little slip that ends up pouring boiling water on any part of me will cause major burns to my hide. Burns that are painful and take a long time to heal. No thanks. Using room temperature water and a bit of dishwashing soap works fine on all of my black powder guns. Even the ones I shoot Pyrodex in.