Execute a custom script when OS X 10.8 is in Power nap?
As far as I know, it's not possible. 

The reason is that Power Nap doesn't have an API that 3rd party apps can hook into (so apps could trigger a wake event or be notified when one occurs).

Whether this will remain like that in the future depends upon Apple. It would be interesting to see if there's a simple way to detect when one is in power nap mode, but doesn't seem to be available afaik. 

If you leave a script that pulls from git every minute, in theory, since Power Nap is really a power on without graphics and audio state, maybe it would get executed (if it was running before). untested.

More info: https://stackoverflow.com/a/11692367/2684