@Mptep, you have some highly confused thinking here. Your external UPS just needs to look like the standard charger, but have a battery in it that is charged from the mains or solar or geothermal or whatever your chosen prime power source is. It won’t run down the internal battery if you don’t make it look like it can be charged from the Framework.
This is just how any existing UPS works. You seem to be making an Everest sized mountain out of a molehill.