Here are some photos of my O2 SCP1.1 with integrated Li-PO battery (7.4v, 230mAh)
External JST connectors for this first prototype (you can connect the original cable)
External switch to upgrade firmware (try to disonnect a JST with only 2 fingers ! )
Very small and light Li-PO battery inside the handle of the controller
You just need a Li-PO balancer/charger to charge/discharge the battery
All the components are avalaible other the internet
Next upgrade will be the internal power switch, but with this first solution, no change has been made on the controller except the JST connector instead of the original cable... for a wireless hardware.