Solution
The guard behavior is not impacted by the output mode (DC voltage vs. DC current) nor whether the user is interested in measuring voltage vs. current.
The amplifier that drives the guard pin is always monitoring the voltage at the HI terminal (measured internally to the device just ahead of the output disconnect relays).
So if you were to measure the voltage between the guard and HI terminals you should always see something very close to 0V. There is a spec called guard offset (in Guard Output Characteristics) that covers how much this can deviate.
But generally speaking, you don't need to worry about that unless the insulators of the cabling are very poor quality.