Hi Jem
Highly recommend the DC-DC route, makes it so much simpler
Here's my version of power meter
I'm using a 3watt +-15Volt unit though, I needed to use a slightly larger DC-DC converter as I'm running the display of the +15V side
For the battery aspect it has a 2500ma Lipo cell connected to a Sparksfun LiPo Charger/Booster http://www.sparkfun.com/products/11231 that handles the recharging of the LiPo cell from either a micro USB or an optional 5V supply and also steps up the LiPo voltage from 3.7V to 5V, it also include a low voltage drop out so you can't damage the battery. Makes it simple
It can also be powered from the USB port so if you do happen to flatten the battery you can still use it while it's recharging
I'm just looking for a nice case to fit everything into and then I'll make a PCB with it all mounted on to finish this off
Cheers
Troy