Interestingly, from my talks with Finnish pilots, they told that the American built fighters did have relief tubes, others not. Those that didn't have included Me 109, MS 406, Fiat G-50 for example. On the other hand, Curtiss Hawk 36 and Brewster did have pissing tubes.
Bombers, perhaps surprisingly, didn't seem to have those, though. Blenheims didnt. Ju-88s/Dorniers, I don't know. British had interesting experiences with jammed bomb days until they noticed that the crew werepissing in bomb bay - and the piss froze the bomb doors, that wouldn't then open...
Or perhaps the tubes on the earlier B17s were poorly located, so the crew thought it easier to do their needs elsewhere.