to mean "she was sat on the floor", in the sense someone placed her there, the "proper" usage is "She was seated on the floor." if she chose to do so herself, either "she sat on the floor" or "She seated herself on the floor"will do, depending on active or passive voice. I know no americans which would say she "was sat", it just *sounds wrong*. but as informal usage becomes accepted as "proper" usage, it may come to be accepted , at least in British English, if not American English.