No register trick, I’m afraid. As far as I remember the sleep mode table in the 328P datasheet, the analog comparator is only a wake source in idle (and ADC noise reduction), not power-down. In power-down only the external interrupts, pin change, TWI address match and the watchdog wake it. Your idle current is the clocks running.
Move the comparator outside. A nanopower comparator with push-pull output into INT0 (D2) lets the 328P sit in power-down at a few µA. TLV3691 draws about 75 nA, and the reference can be a micropower 2.5 V shunt. Add hysteresis with a resistor from output to the + input, a few hundred mV at the battery side, otherwise a battery hovering at 11.8 V will wake you a hundred times a night as the load changes.
attachInterrupt(digitalPinToInterrupt(2), wake, FALLING);
set_sleep_mode(SLEEP_MODE_PWR_DOWN);
sleep_enable(); sleep_cpu(); // wakes on INT0 only
// on wake: read ADC once for the real value, send it, sleep againAoife: you can use the bandgap internally, but the comparator itself still can’t wake power-down, so it doesn’t help here.
On the Hermetry side I’d still send a heartbeat once a day with the voltage. Then an offline alarm on the profile tells you a sensor died, which a wake-only design never would. Put the low-voltage alarm rule on bat_v with some hysteresis too.