Very interesting topic as a theoretical discussion.
I really don't think that you could improve efficiency with a hydrogen supplement system. Adding hydrogen to a gasoline combustion will assist in burning any fuel that would otherwise be sent out the exhaust (not much on a modern vehicle), and will provide some energy simply from it's own combustion. Agreed? Now, of course we can all agree that it requires a good bit of energy to break the Hydrogen-Oxygen bond of the water. The debate seems to be where the energy comes from to break that bond.
The alternator of most common vehicles puts out 60-100 amps @ 12v but then there is a voltage regulator that is generally set around 13.5v. The claim appears to be that alternator doesn't care if it is sending electricity to the battery or not, it still robs the same amount of energy from the engine, and that you may as well use that energy to separate some hydrogen and run it into the intake manifold. Thing is... With an alternator as soon as it detects less than 13.5v on the battery side it increases current to the armature. This raises the power of the magnetic field which in turn forces the alternator to produce more electricity also making it harder to turn.
The only way this should even be debatable, is if someone says that the benefit of the added hydrogen is greater than the benefit of reduced drag from the alternator. Seeing as how the energy goes from chemical potential (gas) to kinetic(engine) to electrical(alt.) to chemical potential(H2) to kinetic (engine) and none of those conversions is 100% efficient... I don't see how the hydrogen system could possibly have a measurable effect, let alone the 15-30% that some of these kits claim.
The only places that you can get "free" energy from your vehicle are from the places where energy is "wasted". The main waste from your vehicle is heat from combustion and braking. So I can see regenerative braking being an option, and I've never seen it mentioned, but why could you not theoretically use a heat to electricity converter (thermocouple, thermionic converter, etc.) to get back some of that heat that would be dispersed to atmosphere otherwise?
The problem with even those types of systems is that you then need a process to turn the electricity back into kinetic energy by adding a whole electric motor setup (hybrid) or a hydrogen system like we are talking about, and again, each energy conversion has losses due to efficiency and it becomes cost ineffective.
If you want mileage, inflate your tires a bit more and evenly, keep off the skinny pedal, use synthetic fluids, keep your steering aligned, check/replace your bearings/hubs... The list goes on and on.