Darrin,
My main concern is not knowing the condition of the tank. I suppose that we could figure that out by accessing one of the FERLL access points and inspecting the outside of the tank. You are right, though, that without a working pneumatic system, we would need an alternate means of "blowing the tank", like a honey dipper, which requires a modified fitting of some kind. If we go that route, then we would definitely bring a sani company out ahead of time to determine what type of fitting that they would prefer.
Still, I like my idea of getting the tank to drain automatically into a ballast tank, since we will probably only ever have gray water waste on board.