I think this would also make another possible fault more dangerous: a break in neutral somewhere between an AC device and the panel. If there was also a DC dev

I think this would also make another possible fault more dangerous: a break in neutral somewhere between an AC device and the panel. If there was also a DC device on the same side of the break as the AC device, then you'd have a possible return path for the AC current that goes through the DC device, the ground wire, and the DC power supply. In addition to that, would it also cause problems with GFCI? The 12 VDC return current would be on the neutral. I'd guess that a steady DC current on the neutral wouldn't cause problems, but whenever a DC device is switched on or off, or you use a DC device that has a fluctuating instantaneous power need, that should induce current in the GFCIs sensing coil that won't be canceled out by an opposite current induced from the load wire. I don't know if that mismatch would be long enough or large enough to trip the GFCI.