Partially taping up the fan is an acceptable workaround. Even with the fan completely stopped or blocked up, the camera stays at a safe, albeit uncomfortable-to-hold, temperature.
I believe what is happening here is that the low temperature is causing an offset that's resulting in a black scene clipping to zero, which it shouldn't do. This would cause black cal to not completely remove the horizontal band, since it was all clipped to black when the lens cap was on. I'll look into this, it should be a simple software fix of adjusting the ADC offset slightly higher.
The sensor does have a temperature sensor, so it will be possible to automatically correct for the offset variations with changing temperature once we have time to implement the fix.