I'm fairly sure I can get something that fulfils the previous log's specification using the Pi Supply: http://www.pi-supply.com/product/pi-supply-raspberry-pi-power-switch/

I think I'll need to cut a usb cable and split the power out to the pi supply. Then the data can go through to the Pi. The power will go both to the battery and the Pi Supply. That way when the unit is plugged in it will charge the battery.

I'm still unsure of whether I really need the Pi Supply. I might be able to just solder some wires onto the power switch of the battery, then the Pi can turn it off.