doubt it would void the warrenty according to the law http://www.semasan.com/main/main.aspx?id=60128
but on another note, i doubt it would work
Well, the electricity has to come from somewhere - energy isn't free. They downplay the amount of electricity needed to break the bond between the H and O moloecules, but I promise you it is significant. Running a generator large enough to do this will place a big load on the engine. Either that, or you will be carrying some big batteries around. Hydrogen has a very low BTU content, so burning it as a supplemental fuel will not recover the energy needed to get it from the water.
This is the main reason hydrogen fuel cells are I long way from being practical. A fuel cell takes pure hydrogen and combines it with oxygen, the by products are water and electricity. Hydrogen IS very plentiful, the problem is it isn't pure, its all locked up with oxygen in the form of water and water vapor. You take water, add electricity and you get pure hydrogen and oxygen. Sound familiar? A hydrogen fuel cell isn't really powered by hydrogen, its powered by the electricity it took to break the hydrogen away from the oxygen. When you recombine the H and O, you get the water and electricity back.
Hydrogen is an energy carrier, not an energy source. Energy must first be put in before it can be recovered. Until we perfect 100% conservation of energy (not possible according to Einstein) this is a losing game. Ignoring the laws of energy conservation makes it a net zero at best. Think of the wound up rubberband that powers those little airplanes. It isn't an energy source, it only transfers the energy you put on it when you wound it up. The hydrogen, when still locked to the oxygen molecule, is just like the limp rubberband.
water vapor is also more effective greenhouse gas than is carbon dioxide.
And they call it HHO or Brown's gas. Look up Brown's gas on wikipedia, it even addresses the fraudulence of these "burning water" claims