The most important issue one needs to consider when approaching this is that we're dealing not just with the backdoor, but the entire waste management system, which includes when Aunt Flo comes to visit Ladynauts. Most current methods of disposal -- the kind truckers use on long hauls, or maybe the crazy shit you use when playing extended bouts of online gaming -- rely on gravity to help us out. If you attach a funnel and piss, gravity will sort it right down the tube into the baggy you attached to your ankle. Not so in a zero-gravity situation, where NASA assures us that not only do solids and liquids like to float about, but they also have a tendency to bond with the closest surface area. Meaning your blob of floating piss is going to stick to your thigh like a sentient Jello mold trying to hug you.
You have a handful of potential options at this juncture. Maybe a mechanical application that mimics what a hand could do -- something that wipes and pulls away. Perhaps something entailing suction, which forces your goo in a certain direction. Potentially some kind of pressure attack wherein jets of water or air are used to remove unwanted fudgkins, but that's probably best combined with suction.
The spacesuit itself is pressurized, and we use this to our advantage. Air is being pumped in at all times, at a rate of 4.5 cubic feet per minute. 0.01 cubic feet per minute can be spared (safely) over a period of three minutes. We simply need to line the astro-pants with sensors that detect notable changes in temperature and moisture levels. Once you start flowing, the system pinpoints the location based on sensor activity and roars into action. To prevent the already-present airflow from rocketing poop up the back of your neck, a simple mesh layer at the waist and thighs will ensure your space sins remain in your demilitarized zone.