While I agree with you about the folly of storing stoichiometric oxy-hydrogen in anything bigger than a party balloon I don't really agree with your remark about fuel consumption, especially with Diesel engines that are past their prime. Hydrogen is explosive at very low concentration (2% from memory) and adding oxy hydrogen can lean out the mixture to the point where fuel consumption drops by up 40%. But you do need to remap the engine control software to do it with any chance of success. In those old Diesels it will also improve combustion problems caused by worn injectors losing their swirl patterns.
But overall, nothing is for nothing of course, and you certainly compromise generator output, but considering that modern alternators are 90+ efficient and that good electrolysis is 60% efficient the 55% efficiency of the HHO system couples with the fact that you can run the fuel mixture very much leaner (I know, i have done it) means you produce a cleaner running system with lower emissions and lower 'store-bought' fuel consumption. If you do it right you might lose as little as 5% overall, but it takes skill and time to get there.