As a Flight Crew Training Instructor on the Airbus, and the B777, I can assure you that it is NOT possible to deploy the thrust reversers whilst these types of aircraft are in flight.
There are numerous strain gauges, squat switches, proximity switches and microswitches mounted on the gear legs and oleos, the signals from which are normally sent to a ground/air sensing computer. Once the gear legs are compressed upon landing, the interlocks unlatch, and the reverse thrust buckets or translating sleeves are allowed to activate.
Hope that helps.
