I'm sure it's possible to design a safe "pass-through" system but it would be quite complicated if it were to cover all possible problems. Complicated = costly and I'm sure that is why it hasn't happened with powerbanks. Such a thing would also require a "system" approach where you would not be able to change any part of the system including the load (which here is dashcams) without adjusting the system parameters to meet the change. So it would be impractical at a consumer level anyway. Ravpower is probably as close to this as we will ever get. For those wishing to design and use a system to keep cams going when parked from other than the car battery then also charging while driving, it wil be safer and easier to use a different battery technology that has less critical charging and usage parameters. The only real advantage of powerbanks here are that for a given energy level they are smaller and lighter, and already have the needed ports, regulators, and charging systems built in. They are not always the best or only choice, just the easiest one.
Still waiting for someone to build us a cam with a miniature nuclear power plant built in so we can do away with all the cabling and wiring problems. Could probably eliminate the need for IR illumination at night then too
Phil