Suppose that you have a language, let's say it's SVO, has a clause pattern in which the subject typically stands for an agent or experiencer and the object typically stands for a patient or stimulus, and in which neither NP is overtly marked for case.
Now suppose that the language has a different clause pattern in which the first argument in the clause takes an overtly marked case, let's call it "case-B," and the subject typically stands for a patient or stimulus while the object, which has no overt case-marking, typically stands for an agent or experiencer.
This second clause pattern is like the passive, except that there's no valency reduction, and the second argument must be an indefinite pronoun if its referent is to be unspecified.
My question is, is there a natural language with such a scheme?