Glider pilots have given this matter considerable attention since once you get really serious about the sport and take on the challenges it presents, long flights are almost always involved. In the early days you simply hung on and drank as little as possible. More recently, adequate hydration is the go, and so more attention has been given to the technicalities of getting rid of the end product. Goog' provides quite a useful series of writings and discussion on the matter:
https://www.google.com/search?client...lider+pee+tube
(Stop laughing, this is serious ... )