Will this work in principle - I could prototype it, but it's not trivial and hopefully 'Freaks will come good (yet) again...
ATMega 168, if it matters....
1) Set the analaog comparator reference to the bandgap
2) Replace AIN1 with a normal ADC port (say ADC0) by using the the ADC multiplexer (so that I can use the AIN0 pin as a normal digital I/O)
3) Use a potential divider on the raw side of my supply that gives 1.1v at ADC0
...and I should get a analog comparator interrupt when the raw supply goes sufficiently low.
That's hopefully the straightforward part.
But now I want to use ADC1 in my app to read some other analog signal.
So I change the ADC multiplexer to use ADC1 at the appropriate time...
...does the AVR remember that the analog comparator is always on ADC0 so that you get a comparator interrupt on low supply even though you might be in code that is dealing with ADC1 when the supply fails?