Technically, in the old days, they used to use animal dung as a tanning agent after liming the hides. They actually didn't use chemicals at all back then, besides what came from the human body. Disturbing, if you think about it, but true.
I don't see them adding the ability to urinate into a bucket any time soon, though. On the plus side, animal droppings are already implemented. Maybe that could be a future option.