Sorry to revisit hydrogen as a fuel, but I remembered a
fuel comparison chart that I downloaded from the Dept of Energy's Alternative Fuels Data Center website. It showed hydrogen with higher energy density than natural gas, but less than propane. So, hydrogen (and by extension, hho) is a fuel, but not a particularly practical fuel.
Much discussion of producing hydrogen relates to unneeded electricity at base-load generating facilities. That would make economic sense, as the energy generated would otherwise be lost, but improved energy storage would probably be a better option (two unicorns in one paragraph).
Back to the topic, the article, "Effect of hydrogen-diesel fuel co-combustion on exhaust emissions with verification using an in-cylinder gas sampling technique," mentions that injecting hydrogen into the intake diluted the oxygen in the combustion mix. I wondered if the oxygen in the hho would be sufficient to compensate for that dilution?