Edit: Just had a check and our meter is one of those where the numbers rotate (like an old digital clock) rather than one with a rotating disc.
Do you think a reflective optical sensor (or rather half a dozen of them) would be sufficient to pick up that movement? I'm guessing it probably would and I could simply count which of the sensors is switching; units, tens, hundreds etc
You only need to monitor the lowest digit. Increment a counter each time the lowest digit switch. If you have access to the digits you could mark all odd digits with a white dot and all even digits with a black dot. Then you should be able to detect whenever the digit switches with an LED and a photo resistor.