I can't believe that my first post on Jacurutu is going to be about Fremen poop- but here it goes:
The books mention that the speed at which Fremen blood clots as an adaptation to the Arrakeen situation. Maybe they have adapted in the way that they poop- leaving almost no moisture in it when they pass it. (type 1 in the apt Bristol Stool Chart above). In that case, the actual mess in the shorts would be pretty minimal after the rocky dooks make their way to the thigh pads. + Maybe there's an easy access clean-out in the thigh pads so you don't have to carry it around with you until the next time you get out of your suit. The transport should in some way be handled by the pumping action provided by breathing that Kynes mentioned, but I can't see how. 'Could be that the thigh pads handling this are in the area of the hamstrings so that gravity does most of it and you just have to jockey it around by hand every time you 'see a man about a horse'. I don't see how the loin cloth figures in...