Check this out:http://www.gammon.com.au/forum/?id=11497
Any sort of external chip is itself going to use power, so I'm not sure if you totally save much. I think I would go with the 8 seconds watchdog, coupled with some of the other suggestions on that page. For one, running at 3.3V rather than 5V saves about 10x the power, so in the milliseconds it is awake it uses less.
You might also look at the fuse settings - at some settings it takes 65 mS to wake up, at others under 1 mS. So the less time it takes to wake up and add 1 to a counter the better.