As i understand it, there are folks in Australia (and elsewhere in the US) who are doing exactly what I was told I could do by Tesla salesfolks (fill up on grid overnight, and sell solar generated power to PG&E) - but because of PGE, there is some rule in place which prevents the storage (w/solar) from filling up from the grid. This is the precise detail which basically rekt my ROI on this system, and gave me a storage system which is near worthless on consecutive cloudy days.
The Telsa forums are full of folks trying to circumvent this silliness (by feeding other power threw the Tesla transducer / current sensors at night, 'tricking' the Powerwall sw to think there is solar being generated) as well as other methods, but your mention of re-wiring it definitely piques my interest. I'd love to have that done, if it's actually feasible - can you elaborate at all on that aspect?
Ill write it in its most basic form:
Disconnect main input breaker to separate you from the grid
Fire up a generator, and link it to your internal wiring to feed it; right at the main panel just after the disconnected mains. This should have the ability to trick the system into thinking you are connected to the grid still.
another way is to simply wire in the powerwall as if you had no Pv(solar) system installed.... ignoring the solar system in the diagrams... then once powerwall is installed, enable the standard grid-tie link.
I do not suggest doing any of this though; because its much more complicated than this.. and I have omitted several key issues that will pop up in the config and wiring of it... you just gotta look at how they hook up and decide for yourself... or get an electrician buddy whom can give you the proper cautions.