I wish somebody would try my "variable displacement" thrust dingbat, which would only be modification of what you show. Here's how it would be done:
Use a hollow cylinder that just sinks in water - say one of the really big CO2 cylinders with enough added ballast to make it neutrally bouyant or just slightly sinking. You'd also need a deep bucket of water, of course. Suspend the cylinder from your cable by threading an appropriate fitting in the neck. Glue a waterproof ruler to the cylinder's side, like a section of steel measuring tape, and calibrate it on a chart using various weights pulling it up out of the water via a pulley. The greater the force, the higher it will rise above the water's surface [which will drop slightly as the displacement is reduced - that's why you want the scale ON the cylinder!].
This is only a little more work than what you have, and once calibrated would give you instantaneous readings at any power level. You should also raise your pulley if possible so that the upper part of the cable is pulled truly horizontally, for good linearity.
Very nice build of this engine, Nick! It will be fun to learn what this does to the thrust, as well as to the observable heating pattern along the pipe.
Great work, as always!