I have a project where I'll be using an XBee RN-XV and chose the Arduino FIO as the host for it due to it being small and low power and having an XBee header built in. My intention was to run it off of USB power and not have a battery. I quickly discovered that the charge circuit was not powerful enough to run the wifi chipset without a battery to assist. In looking for a solution, I discovered (with the help of birdbrainlabs on reddit) that the battery voltage regulator can handle up to 5.5V, so it can handle USB power directly.
I could have added an additional USB jack, but decided instead to modify the board to just get power directly from its own USB jack.
Examining the board, I decided the best option was to cut the trace right at the CHG 5V pad and then add a jumper over to the BAT + pad. See photo's for locations. The jumper wire is on the other side, but you can see the solder blobs where the wire comes through.
Cutting the trace is simple, you can use a dremel, or a sturdy knife. test with a multimeter to make sure you get the whole trace.