Con-pilot can correct me, but the Sabre 65 could deploy the TRs in-flight if they were armed. There was an arm switch on the pedestal; the lot I flew it with would at places like Hilton Head do this "non-standard" procedure.
Boy, you're asking me to wake up some the old gray cells now. Yes, there was a switch that had to be turned on to arm the reversers, it also served as the emergency stow function.
But for some reason I want to say that one of the main gear squat switches had of have been made before the reversers would deploy. I'll try and find my old Flight Safety Sabre 65 training manuals and find out for sure.
Remember, a coupe of Sabre 40s did have accidental airborne deployments. This of course got the FAA's attention and all subsequential Sabre aircraft had the switch installed during its manufacturing and all others were retrofitted.
In any case, I never tried to deploy the reversers in flight, as just before touchdown, no matter how much I was tempted to operating in and out of Aspen.
I do remember the emergency stow procedure, if the throttle auto-snatch (retard)* did not bring the affected throttle (thrust lever for those that insist), you did and then move the reverser switch to auto/emergency stow and hope to hell it worked.
Oh, and just remembered, if a thrust reverser accidental unlocked, the hydraulic system would immediately force it back into the lock position and then (I think) keep the pressure on the reverser until the aircraft landed and shut down.
There were a lot of safe guards built into the 65 reverser system and I never heard of one actually opening without commanded to, with the switch armed.
It always worked in the sim.
* That was a one time only function, once the throttle was advance again, the auto-snatch (retard) function was no longer in the system.