That could be a good way to determine the tailwind. I have done this for 42,000 landings in which I took the difference of the QAR recorded GS and TAS at around 10 m. AGL and compared it to the tower wind tailwind at the moment of landing. Correlation between both was good. The problem remains that during the landing as a pilot you want to asssess the tailwind before you have reached 10 meters AGL.