yes it can be done, and yes it could work.
but the reason we use pure ammonia is that we know it's pure, and there's nothing else going on in it. your pee is not like that. everything you eat, everything you drink, even your daily grooming habits can change the molecular makeup of your pee significantly.. think about it.. have you ever eaten asparagus and your pee smelled funny? drank coffee and your pee turned dark? ever had a urinary tract infection?
personally, i dont want that junk in my tank.
so yes, it could be done, but at a risk of killing/harming the fish.