Dear Daniel, You asked about the range of DC light levels we should expect and the important causes of changes from the nominal dark sky conditions. The sky background may vary due to clouds which can either decrease the background by blocking the sky or increase the background by scattering artificial light from ground sources. The background can increase due to aurora effects, especially during solar max. There are transient effects from meteors, airplane strobe lights, distant lightning, satellite glints, etc. Perhaps none of those transients would be slow enough to count as DC for your purposes.
Bright stars are an important cause of intense brightening of the background light.
The dark sky background integrated over a square degree is approximately equivalent to a 4th magnitude star. For our 1.5deg diamter pixels, the background is closer to a 3rd magnitude star. Thus a 3rd magnitude star entering the field of view would at least double the background. Moonlight, if present, brightens the sky a lot. The Fly's Eye and HiRes prototype operation are restricted to dark nights with the moon below the horizon.