In the very early days of jets, a direct force measurement (strain gage) was attempted but it was fairly useless on buried engines because it did not include inlet forces.
But podded engines are another matter - maybe Mr. A or Mr. B could give some insight whether measuring thrust mount or pylon strain is feasible.
Understand, though, that available physical thrust is a function of altitude - so tables or a corrected readout would still be necessary.
