It occurred to me while the livestream was on that the Pioreactor has a current module: the LED channels!
I hooked up my ammeter between the positive and negative of an LED channel (D, specifically), and 0% "intensity" is 0mA (obviously), 20% is 23.8 mA, and 50% is 55.5mA, etc.. The internal circuit has a feedback loop so the output is incredibly stable, up to two digits on the mA scale.