Hans Vollmer built an adapter that allows to power a camera from a power bank. The power bank has to implement the Quick Charge 3.0 or 2.0 standard in order to work with this solutuion.
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.
Read more about this solution on my battery adapter page.