I think you're barking up the wrong tree. For the z-series Atoms with Polusbo (i.e. your system), the processor is already using less power than the chipset, the screen or the hard drive.
The processor already clocks-down to several layers of sleep states (C0-C6, C6 = 80mw), and I guarantee you can't come-up with an under-volt that will reduce power more than C6 (the processor under-volts and clocks at 800 MHz). You would lose all these fancy C-states the moment you messed with the voltage, so it's really not worth messing around with.
What you need to do is recognize what is responsible for eating the power in your system. Luckily, Intel has already done the heavy lifting with this excellent in-depth article on Atom Z-series + Poulsbo in MIDs
But that read is really heavy, so allow me to sum-up the important facts:
You have two things you can do to easily reduce power in your netbook. First, your screen: you can start by reducing the brightness, get it as low as you can go without discomfort. Next, change your power settings to more-aggressively turn off your screen after just a few minutes idle. Your screen consumes roughly %30 of your total system load (it is a very large screen for a netbook). You can reduce this with lower brightness and quick deactivation during idle.
Next, you need to tailor your system to not put a heavy load on the processor and memory, so that it can reach the nirvana that is C6 more often. One of the biggest consumers of power when on the internet is Flash and other rich websites with lots of animations/effects. You can easily remove the effects of Flash on your battery life by installing Firefox with Flashblock. You can still run the flash things you want to see, just by clicking on the play icon, and you can whitelist entire sites.
The fact is, flash elements are so demanding that the processor doesn't ever reach C6 state, and is constantly being woken-up. the result is high performance drain, even when you're sitting on a site with a couple flash ads.
Still not satisfied with your battery life? Well, now it's time to pull-out your checkbook. If you have the three cell battery, you can buy a 6-cell battery and get over 6 hours video playback
. Or, you can swap-out your hard drive for an SSD, but this will only save you 20-30 minutes of uptime, and costs more than the new battery.