Inboard inflight reverser deployment up to Vmo was approved in the DC8s. Even an asymetric reverser deployment at high speed was no drama.
But definitely not approved for the Boeing model 767 as Lauda Air, 223 poor souls and their loved ones can attest.
Thrust reverser system certification by the FAA required that the airplane be capable of continued safe flight and landing under any possible position of the thrust reverser (FAR 25.933(a)(2)). However, wind tunnel tests and data used in the simulation of this accident demonstrated that aerodynamic effects of the reverser plume in-flight during engine run down to idle resulted in a 25 percent lift loss across the wing. Simulation of the event disclosed that the airplane was not capable of controlled flight unless full wheel and full rudder were applied within 4 to 6 seconds after the thrust reverser deployed.
Fifteen minutes and one second into the flight the co-pilot's voice was heard to exclaim, "ah reverser's deployed," accompanied by sound similar to airframe shuddering, sounds of metallic snaps and the pilot-in-command stating "here wait a minute." The cockpit voice recording ended twenty nine seconds later with multiple bangs thought to be structural breakup of the airplane.
See following link for sobering reflection:
http://www.rvs.uni-bielefeld.de/publ...aRPT.html#1.16