Came across one of those "power banks" that support passthrough charging - that is, it can be charged while also supplying power to a device downstream. It is a Ravpower PB060, which has the shape of large candy bar and weighs around 4 ounces. The good news: it indeeds supports pass-through charging. The bad news: if it loses power from upstream (e.g., stopping the engine shuts off the rear USB ports in the van) while powering a device downstream (e.g., FireTV Stick), switching to battery reserve is not seamless and the short blip power cycles the device, so it is useless in the context of the discussion here. But then it is unrealistic to expect a sub-$20 piece of electronics to behave like an Uninterruptible Power Supply for computers.