Wouldn't it just make sense to let the person choose if they would like to be defined as their current sex or what they are transitioning to? So for example, if the person is MtF and they want the government to call them F, they can just ask the government to switch their sex status.
Having a "T" class doesn't make much sense since it doesn't really model what is happening. T is not in itself a sex, it's the representation of people transitioning from one sex to another. That inherently means it is not a state but instead it's a mutation.