(update below on the L-bracket connection.)
You only need to spin the props up for a few seconds to take the thrust reading. You can then shut down and then read the values off the watt meter.
Update: it's working pretty well, with the exception of excess vibration when spinning big props at high speed. That's because when I drilled the mounting hole for the L-swing I left a tiny bit of clearance so the thrust would transfer smoothly to the scale, but that introduces slop which induces vibrations at high thrust. I've got some 4x10 bearings ordered which will hopefully smooth things up.
Here's the unit in action: