i dont think theres anything wrong with a wee before weigh in, now, during would be unnerving....................
i WI at 7, i try and have lunch at 12 ish, then prob wont eat anything else till after, but i have a normal lunch, today has been a ham salad box, it was quite big, i try not to drink after about 4, im pretty conscious about how much liquids weigh, 1l of water is 1kg, 2.2lbs, so a can of pop or a mug of tea is about 0.7lb !!!!!, now in the scheme of a week thats nothing, as your hydration levels go up and down, but for me drinking it before weigh in would be like some kind of hari- kari