For some reason, books and magazines never seem to cover going to the toilet, and it’s only when you have to go for the first time when you’re wearing a harness that you realize why gear’s designed the way it is (or what a bummer it is when it isn’t). Things like removable leg loops, easily releasable rear risers, fly zippers that can be zipped both ways and the design of your trousers all come into play.
If you have a pair of standard over trousers with full-size zips (or 1/4 length zips), then you can either undo one leg loop and rear risers, then unzip one side of the salos/trousers and pull the fabric across to bare your arse. The other option is to undo the rear risers and unzip both zippers down to your leg loop and squat down (make sure you pull the fabric between your legs as well as your rear risers! When squatting there, just take a moment to think of that story about Chris Bonington having a dump in the hood of his down suit!
If you need to take your harness off completely, then you can larks foot a sling around your chest (larks foot it once, then pass the sling around and through one more time, so the sling is locked).
One of the best systems for dumping is the through the legs zipper, so you can have a crap without taking off your legs loops, making it super fast (at the moment you can only see this on Buffalo Salopettes). The other system is the rainbow drop seat, which is somewhere between the two but does add extra bulk to the back of your pants.
Anyway hope that helps - and maybe you should practice it in your garden!