Since you seem to have experimented with this, I'd like to ask: don't you need a certain "minimum" time to capture the faintest of stars, because their signal otherwise wouldn't rise above the noise floor? I've been curious about this with stacking in atrophotography for a while.
> The other technique is dark frame subtraction, where you take an photo with the lens covered at the same ISO/shutter/aperture as the rest of the set. Digital sensors have readout noise and heat noise which tends to be very predicable. Subtracting this noise gives you a cleaner shot as well.
For long exposures this heat noise becomes more problematic: long exposures heat up the sensor quite a bit. Have you tried building your own customised electric coolbox to keep temperatures down? It can be a fun project!
I actually used this in a different context: during long Skype sessions at home with my people abroad, my phone heats up a lot and the video would start stutter. Putting it on a cooling block actually fixes that. This had more to do with the system not having to scale back CPU performance to let the phone cool down, but a side-effect according to people on the other end of the line, was that the image quality improved too