Frankenstein knife switch
Seems very odd to me that the old multi-blade knife switches like you used to see in the old horror movies would do the job.
I'm quite surprised such a thing isn't made to handle the simple act of dis-engaging the grid and connecting a generator. All that's needed is a DPDT (or 3PDT if you want to switch 2 hots and neutral) switch of sufficient beef to handle the current.
If you search for "Leveton knife switch" you will see what I'm talking about, though its only for 25Amps at 125 volts. I'm looking for a similar solution, albeit modernized and capable of handling 50 - 100 amps at 240VAC.