So I was following my adventuring sim in Al Simhara this week. The moon turned full while she was there and, sure enough, a zombie appeared right next to my sim as she was getting into her tent for the night. Since my sim was not available for a zombie attack, the zombie, a townie named Na’im Tut who was apparently an acrobat (thank you, Sims 3 Showtime expansion pack!), shambled around the building at the base camp and did this:
So even though mummies don’t have to go, I guess that becoming a zombie doesn’t remove the bladder need.