One option I can think of is

1. Prepare two models: The model A has artificial lighting with daylighting control (Daylighting:Controls and Daylighting:ReferencePoint). The model B does not have articial lighting (i.e. change the lighting level in the model A to 0W).
2. You can obtain the Lux at the Daylighting Reference Points for both the model A and the model B by Output:Variable Daylighting Reference Point 1 Illuminance, Daylighting Reference Point 2 Illuminance, Daylighting Reference Point 3 Illuminance...
3. The Output:Variable of the model A is the Lux for artificial lighting + the natural lighting at the Daylighting Rontrol Points. The Output:Variable of the model B is the Lux for the natural lighting only at the Daylighting Reference Points. So, you can calculate the Lux of artifical lighting only at the Daylighting Control Points.

Lux of artifical lighting only = (Lux of artifical lighting + natural lighting) - (Lux of natural lighting)

i.e.

Lux of artifical lighting only at Daylighting Reference Point 1 = (Daylighting Reference Point 1 Illuminance of the model A) - (Daylighting Reference Point 1 Illuminance of the model B)