Actually while hiking at today at 9,900 ft I had an epiphany. My plan has a rather glaring/stupid hole. That accessory ground rod that I dreamt up for good measure will nicely link the inverter ground to the premises ground, where ever that is (likely about 200 ft away at the first main panel) via the good ole' earth. And since the neutral and ground are bonded there, I will surly get ground faults from the inverter. To work, the ground from the Inverter must be isolated. Period
Hammick, as you note the chassis of the Lightning and the ground of the inverter are in continuity. I am even worried that if I drive the Lightning into the leanto with muddy wet tires, there may be enough "leakage" of ground via the wet muddy tires to throw ground faults, even with the ground in the generator cable disconnected. We shall see. If it happens, its a nuisance fault, not something dangerous.
But my idea of a switch is still OK I think, but labeled like this:
Up: Generator lead cord grounded to premises ground. (generator/source w/o GFI)
Down: Generator lead cord ungrounded (generator/source with GFI that cannot tolerate neutral/ground bonding)
I would rather be entirely transparent that I am taking liberty with the code, rather than create some "double secret ungrounded dogbone" that could get mis-used down the road by someone who comes along after I am gone.
The relative safety of such a situation was discussed in detail already, and alternatives, either completely fixing all the neutral bonding on the premises, or installing an isolated critical loads panel are just not an option for me. I think those other options are viable and very reasonable for someone who must remain in strict code compliance, so I am not arguing against those solutions in any way.