I heard a story that Simms had refused to deal with a pair of waders sent for warranty work as the owner hadn't bothered to remove them before relieving himself. He apparently worked on the theory that gortex wicks away liquid from the inside to the outside. Something to think about if buying secondhand waders.
I've never had much bother coping with a No. 1 while wearing chesties. I did have a particularly regrettable incident many years ago at Portmore when enacting a 'bear-in-the-woods' manoeuvre! I don't think the fitting of a waterproof zip would have provided the answer there.