Thanks @chrisys! That clears up the confusion. There are several USB power bank products that support pass-through charging, which is why I think of them as performing the same function as a UPS or the PiJuice. You’re right that most don’t, but some do.
Regarding the use case—yeah, good idea.
The intended use case is a server for a medical record system at a remote facility (possibly even a mobile clinic or temporary structure in an emergency context) that has only unreliable mains power or a generator. We are trying to maximize the ratio of battery capacity to system power consumption; the Fin is attractive as a low-power server. And we want the parts to be easily sourced and replaceable; hence USB power banks are attractive as mass-produced products.
We are also seeking an overall configuration that is simple enough to be communicated to IT staff (not engineers), such that they can easily set it up correctly and, should a part malfunction, easily obtain and install a replacement part. Thus, we’d love for our instructions to say “plug this USB power bank into your power source, and plug the USB cable from the Fin into the power bank”—which everybody knows how to do because everybody has a phone. There are only two parts, the power bank and the Fin, and anyone can figure out how to put them together. If they lose the power bank then they can just use a wall charger or a non-passthrough power bank as a stopgap; it will even run on a car charger or in an airplane because there are 5-volt USB sources everywhere and everyone knows how to use them. We’d be pretty nervous if our instructions sounded like “take this naked LiPo battery and connect these tiny cables to some pins on a naked circuit board, mount that onto another naked circuit board and be sure to line up the pins correctly or you might destroy it, put the circuit boards in a case, assemble the case with four screws and a screwdriver, then plug this special DC adapter into the wall and plug the cable into the barrel jack, and if you lose the DC adapter, you won’t find another one within a 100-mile radius.” Especially if they don’t speak English.
A device with a micro-USB port is something that almost anyone in the world can plug in and power up with zero electrical engineering knowledge. A device with a DC barrel jack can only be powered up by someone who has specialized knowledge of how to get their hands on a DC adapter of an acceptable voltage with a cable whose plug has the correct inner and outer diameter to fit the barrel jack.
I hope that helps paint the picture! Thanks for helping us figure out the best way to do this—it has the potential to really help some people in need.