The N1 decay rate is known (FADEC chop), the efflux is known, the effect on the wing devices can be modeled along with the lift vs aircraft speed with variable rudder inputs.
All that stuff was done to certify the 767 and meet 25.933 pre-Lauda. They even did a flight test - IIRC at a steady 200 knots/20k and the engine at idle, they deployed the reverser and showed it was controllable.
What Lauda showed us is that the analysis was garbage - it didn't reflect the real world. Unless the engine was already at idle when the T/R deployed, the aircraft wasn't controllable. So, rather than try to figure out how to fly an uncontrollable airplane, the solution was to make sure it never happened again.