Hey, 5T. Any intermittent current/voltage fluctuations at the rotor/brushes will not induce enuff stator output stress to affect the rectifier.
To do this "brush current" test correctly would require 2 meters.
1 meter measures the voltage across the brushes, the (+) probe to the positive brush, (-) probe to the negative brush.
The second meter is the ammeter, measures the current thru the brushes. Could be done by disconnecting the positive brush's lead/lug, will need to put the screw back in there to hold-down the brush, and connect the ammeter to that brush and it's disconnected lead/lug. Could also do this at the regulator connector, remove the male/female positive connectors from the connector block halves, and connect the ammeter to those 2 connectors.
Connected like this, then you simply divide the observed "across brushes" voltage by the observed current, which will give you the combined resistance of the brush/slipring contacts and the rotor.
It's also really helpful to have a 3rd meter measuring the battery voltage, and note all 3 meter readings simultaneously.
Edit: The catch is, I'm not sure if Daniel measured the brush current, or the battery's charging current, a totally different thing...