Like Bjarney is mentioning.(didn't view the linked item, but figure it's a supplimental connection with - possibly OCPD - which will allow a 3 wire + ground connection).
If the 240 VAC receptacle's EGC tab is connected to the winding's center tap directly (not just to the frame), then an EGC could be bonded to the frame and the receptacle could be swapped out for a 3 wire + ground receptacle.
For the absolute simplest (and "cheap-o") method, just plug in needed loads during power loss! Unplug them from normal power outlets, and plug them into the Genny.
This is a non-efficient, yet effective method; but many extension cords are required!
Just added this to give the alternate option, not to be a jerk!