Wouldn’t it be as simple as to sense if there’s ENOUGH or none power at the powerpack input, so a relay (or solid state circuit) switch the pack output to the device being powered to be powered from the pack, and the power input disconnects. By the same token, when there’s power, it’ll disconnect the pack conection to the device, and the charger will power both the pack and the device? That’s in essence what we do manually with the pack; when there’s energy available, i power the device AND charge the pack with the charger (well, there are two chargers, one for the device and one for the pack, but a powerpack charger can integrate both), so when there’s no power available for the charger, I power the device with the power pack and disonnect it from the charger.
I have the need to power a wireless camera/cellular hotspot combination that has an unreliable supply of energy, so the solution I HAD to install is a small UPS to power the camera and hot spot adapters, instead of a dual output pass-through powerpack that may be a more elegant and cheaper solution.