"Listen to Pope, Folks." Love it, haha.
The thing is, the hydrogen generator would help a bit if you set it up with a throttle switch, so it injected the gases on demand, such as climbing a hill, or passing. The power loss from the alternator would be small, over a long period, and the burst of power would benefit a smaller engine. But do you want to drive around with a quantity of highly explosive hydrogen/oxygen mixture somewhere on board? Not me, thanx.
Totally different from a hydrogen cell, btw. A cell contains no oxygen, and won't go BANG!