Is it practical to install seat belts in airline lavatories?
While it may appear at first glance that a seatbelt could simply be installed, it is not a simple answer. There would actually have to be some significant certification analyses and testing done. Seat belts tend to be fabric. While those fabric lap belts are approved for the rest of the cabin, the lavatories have different certification requirements especially regarding propagation of a fire in the enclosed space. There would have to be fire certification testing done and analyses to verify that the information gained during the certification testing was applicable to all configurations. 

Also, who would want to use or even touch a lap belt that was installed in the lavatory while someone with explosive diarrhea experiences turbulence? Eww! I wouldn't want to use it as a passenger or service it as a mechanic. That is a biohazard.