How can I enable and/or disable OS X's power nap feature from within Terminal?
Use pmset to change the darkwakes parameter:

sudo pmset -c darkwakes 1


The darkwakes parameter is not mentioned in the man page, but it is shown by pmset -g if your Mac supports Power Nap and is operating on an AC power.

-c is AC power (or charger or wall power), -b is battery power, -u is UPS, and -a is all.

You can also edit the plist directly, but you have to restart to apply the changes:

sudo /usr/libexec/PlistBuddy -c 'Set "Custom Profile":"AC Power":DarkWakeBackgroundTasks 1' \
    /Library/Preferences/SystemConfiguration/com.apple.PowerManagement.plist


Or using sed:

sudo sed -i '' '/DarkWakeBackgroundTasks/{N;s/false/true/;}' \
    /Library/Preferences/SystemConfiguration/com.apple.PowerManagement.plist