The flaw in that theory is the 120 would not saturate the core of a 240v primary and your power would end up being very low. I have had that problem in the past trying to use 240v transformers on 120.
I was really thinking more along the lines of tricking up the voltage regulator in the alternator like those old JC Whitney boxes that gave you 120v from your car.
I know an alternator guy and I was going to ask him. I think you need an external VR to do it.
