I finally got the various pieces working and collecting live data. S3-R1-A/current appears to be the key value, measuring the photodiode output in micro-amperes. The vis value is the voltage drop in volts when the current passes through a 390 ohm resistor (vis = current * 0.00039). S3-R1-A/illuminance represents a lux value and is calculated from the current * S3-R1-A/gain (defaults to 105.263 assuming an SFH203P Osram photo-diode).
Not having an accurate lux meter, I have no idea what the right gain value should be. I was getting illuminance values ranging from 3000 to 9000 around noon today under varying cloud cover, which seems a bit on the low side.