auto-recalibration of a power supply through something stable over time? (thought experiment)
How far you have to go depends on how accurate you need.  For many applications, a zener diode is sufficient.  The next level might be a zener at a specific temperature, held there by a heater.  There are also "reference" cells, which are really batteries that as little as possible current is drawn from.  These need to be at the right temperature too for the best accuracy.

If you keep going, you have to worry about the temperature sensor drift used to keep whatever you are using as a reference at the right temperature.  Then you have to worry about how exactly you are comparing the reference voltage to something else.  Every means of comparison will have some error, which will drift over time, be dependent on temperature, etc.  Even more extreme, you have to think about thermocouple offsets due to different parts of the circuit being of different temperatures and made of different materials.

Without a number of how accurate "good enough" is, it's impossible to say how far you have to go, how much you have to spend to get there, and whether it's even possible with current technology.

Even if you have a perfect power supply, what about the voltage drop in the wires between it and the load?  You have to look at the whole picture and think carefully about what problem you are really trying to solve.