IIRC in flight deployment was not physically prevented with the external clamshell reversers, but their use was prohibited in flight.
That is what I recall as well, once in recurrency school we got into a discussion on just why there was no physical prevention to opening the reversers in flight and the instructor, an old 727 FE since day one of the 727, said that when Boeing made the 727, they figured pilots were smart enough not activate the reversers in flight. But, if for some reason if a pilot thought he really needed to, Boeing made it to where if they had to, they could.
Now, just how true that is, got me, sounded damn good at the time.
Oh, the School was Dalfort, the former Braniff training school.