Could the toilet be used as an extra passenger seat? In the US, if the toilet had a seat belt, would it be legal for an airline to board one more passenger than the number of normal passenger seats, requiring that whoever used the toilet last must remain there until the next person needs to use it?  

Inspired by: "A new engine inlet reduces vibration and cabin noise, the lavatory receives a belted seat allowing a fifth passenger and the Garmin G3000 gets computed take-off distance and stability/angle of attack protection."  -- HondaJet Elite

Related Embraer Phenom 100
When I was with Weber Aircraft we provided many lavatories for Boeing aircraft.  There was never any requirement for a passenger in the lav during structural tests/qualification.  Based on this I know that it would be illegal for have someone in the lav during takeoff or landing as it would exceed the structural qual test requirements.