I performed tests to power Olympus cameras (OM-D E-M1) from a 5 V source. The voltage of 7.5 to 7.6 V required by the camera was delivered by a DC-DC step-up converter.
In general, powering a camer this way is possible. I just couldn’t find a power source yet that allows to drive the camera without restrictions. Even a wall wart delivering 2 A maximum current wasn’t enough to get single frame pictures at a decent pace. Too short image intervals caused troubles saving the taken pictures. Common USB chargers won’t drive the camera even in idle mode.
Maybe a power source delivering 2.5 or 3 A may do the trick. I couldn’t verify it yet.