In reply to pancakeandchips:
I agree the more environmentally friendly solution would be to climb the route in one go and not bivi in the middle (such that you might well need a crap on the route). If a bivi is planned on an area of very limited space, Wag bags are simply the best solution I know of. Packing out your shit is compulsory on some big walls (eg Yosemite).
Normally, responsible people who can chose a sensible place to shit when wild camping carry a small trowel to bury it.
The issue isn't just about environmentalism, its also about health. Toilet health is a wider problem some people just don't seem to get: like morons I've seen having a crap next to a stream that feeds into a water source (eg some Cuillin ridge aspirants who bivi in Coir' a' Ghrunnda).