If you have enough excess fluids that you need to pee, you are obviously not working hard enough
.
I all seriousness, when I did my first 100 miler (actually 97.3 as I found out later when I took out my phone at home
), I drunk about 5 litres of fluids (including a coffee at my lunch stop) and never had a wee, or felt that I needed one, all day. Someone on here suggested that I had gone into something called "Ketosis". Dunno if I did or not, but it was odd to drink so much and not need a pee.
But when I do need a pee, I would never consider doing the act in the saddle (well not deliberately). I did have some wee come out once when getting a serious wobble on when going down a hill at 40+ mph, but I don't think that counts.