Hi RG2019,
As far as I know, nobody has done this. Zero installs either a Charge Tank or a Power Tank, but not both.
a. I don't think so. The signal harness most likely provides CAN communication with the bike. To make it work, you'd have to figure out how to turn on the charge tank with a different controller. Much like the Diginow now does with their own chargers.
b. The powertank signal harness is also used for CAN. You'd have to get your powertank to close the contactor with an external controller. It would probably really mess with the SOC and range calculations, and perhaps the isolation fault detection system.
c. If the signal harness is just CAN, you may be able to join them together, but I have no idea what the bike's firmware will do with it. And we have no way to modify or look at the firmware
It sounds like the most tested way to do what you're asking is to use a dignow or other fast charger with your power tank. Plenty of people have done that. Or maybe get an SR/F with a powertank and 6kW charging when it comes out later this year.