As I am new to Stack Exchange, I happened to see the question only now and mine is a belated answer. If Mark is still interested in an answer, then:
It is a good question gaining added strength with the EDIT.
For thermodynamic analysys, a process must connect two equilibrium states A, B of a system. Take a state C of the system on the way from A to B. We can now consider the process A to C or C to B. To qualify for such a consideration, C must satisfy the condition that it is an equilibrium state of the system. C being chosen arbitrarily, it follows that every state of the sysyem along the path A to B must be an equilibrium state.
Coming to the EDIT:
To simplify the discussion, let us assume the syatem to be an adiabatic system. Suppose A is an equilibrium state and B is not. Then, let the system go from A to B reversibly (if possible), then the entropy of the system (and of universe in this case) decreases. Therefore, the process B to A becomes a spontaneous process. We can then employ the spontaneous process B to A to do work for us. Therefore, we let the process go reversibly from A to B, then let it go from B to A spontaneously, doing work for us. We can repeat this process endlessly and perpetually extracting work (energy) from nothing! We thus achieve perpetual motion if an adiabatic system goes from an equilibrium state A to a non equilibrium state B reversibly.