This mod has been attempted before, there is even a controller mod store that sells pre-made internal battery controller for the 360 with lipo battery pack.
All PS 3 controller use and run lipo battery and can charge them. So you can attempt to do this. I personally has always wanted to do this and from what I do know you need two things
1. PS 3 lipo battery pack (ebay sells them cheap, if you have a 3rd party knock off of the ps3 controller they also has a mini lipo pack)
LINK2. Lipo mini recharge board with mini usb port
LINKAs I said this has been tempted but no one ever said if it worked or not. Seen few mods with this being done not sure why not everyone is doing it. This is basically all you need. Not sure if you have to retrofit the controller and eliminate some connection parts off the board or not.
Here is a quick video of how someone did the mod using a standard rechargeable battery pack for the xbox 360 controller.