the dc9 series 30 can (but is not authorized to) deploy the thrust reversers in flight. just pull throttles aft, and pull reverse levers up and back.
NOW, again YOU ARE NOT AUTHORIZED to do this!
We train for uncommanded thrust reverser deployment in the sim. if unable to stow the reverser, you shut down the engine and fly on the remaining engine.
I've asked if the reverser in the span air crash case deployed in flight, but I have no data to think that it has happend in this case.
The douglas dc9 series of planes have demonstrated inflight thrust reverser deployment and the ability to fly the plane.
boeings do it with mathematical models and theory.