Actually it may not be a bad idea. Using the gas engine's generator to electrolyze water into Hydrogen and Oxygen and inject both into the air/fuel mixture to enrich the stuff and thereby getting more mpg. Also there wouldn't be any dangerous storage of Hydrogen as it is created just as it is injected. Pretty smart, actually.
Giving this more thought, I suppose one would have to realize that the laws of Thermodynamics haven't been repealed. The energy it would take to hydroyze the water would put extra strain on the engine requiring more gas use which would offset the gain from getting energy from Hydrogen in the cylinders. What with friction loss the engine would actually get less MPG than otherwise. Sort of like a Nigerian scam e-mail.