Finally, I found a DC-DC converter that is powerfull enough to drive a camera from a USB powerbank. The powerbank has to provide at least 3A at 5V to satisfy the power consumption of the camera during shutter release.
The converter is small enough to be built into the adapter on request.
After a series of tests I have built a BLH-1 battery dummy for Olympus cameras (currently the OM-D E-M1 Mark II). Since the battery is larger than other models, the lid of the battery compartment is also bigger. This allows to cut a slot for the cable to pass through while the lid is closed. You shouldn’t do this to the original lid. That’s why I currently try to get spare battery compartment lids for the camera.