I think the most viable type of ADC with a bare comparator is to measure the time while a capacitor connected between comparator and GND gets charged via a resistor from a defined voltage (e.g. full discharge - switching the comparator pin to output momentarily and settin it to 0) to the comparator's trip voltage.
OK, it's kind of slow, but that depends on the application, if it's sufficient or not.

I think there is an appnote on Atmel's website for AT89C2051 (which has a comparator and no ADC), if I recall it right, it's for a thermometer.

Jan Waclawek