Having machined various metals for the last 30 years, I just don't see how this is going to work. Machining any metal stresses it. The cutting conditions determine the stress added. Stressed metals behave differently based on how much they are stressed. You can work harden parts just through machining.
When machining batches of parts, we can't even rely on the material coming in to be consistent. When sending machined parts out to be hardened or stress relieved, we can't be sure how they will move.
When relying on such small amounts of flex to measure power, I don't see how they will be able to control the quality of the product it is going in to a close enough spec to create accurate results. Maybe each person will have to calibrate it, but how, or to what?