The charge tapers approximately the same amount of time in both hilltop mode and full charge. I don't see the taper pointing to the amount of "reserve" battery.
Why does nobody think you can easily retrofit DCFC?
Here should be the required parts:
DCFC Charger = 24285895 or 24286351
DCFC harness = 42555649
DCFC receptacle = 24287239
You can readily buy everything except the harness. Also, the charger is on top, so its very easy to access.