GoPro Hero 2 Battery Elliminator? (3.7v versus 5v)
According to Benlo.com:

Figure 1. Hero 2 battery eliminator connection points.

Figure 2. Hardwired Hero 3 battery eliminator.

Using a Battery Eliminator with the Hero 3 Camera
GoPro cameras can be run off a 4 volt power source which replaces the internal battery. The voltage must be no less than 3.75V and no greater than 4.2V. The power supply must be capable of supplying 0 to 1.1 A of current without deviating from the above range. The T-sense pin on the camera is not needed with a battery eliminator power supply.

You can't supply directly from USB 5 V supply. Some form of regulation will be required.



simulate this circuit &ndash; Schematic created using CircuitLab
Figure 3. Simple &quot;constant&quot; voltage dropper.
You could try the circuit of Figure 3 to power the camera. The theory of operation is that

each diode will drop 0.7 of a volt when even a few mA is drawn through them.
R1 draws the small current so the voltage across the resistor should be about 3.6 V.
Depending on your exact 5 V voltage from the USB port, the diodes and the camera this might just work. It won't cause any damage as it would be the same as a flat battery.
If it doesn't work then you could replace one or both diodes with Schottky diodes which have a lower forward voltage drop.

Procedure:

Connect up the components but not the camera.
Plug in to USB port and measure voltage across R1.
If voltage is about right then unplug USB, solder up and test.