Ok. I think I'll shoot the final bullet into the dead mythical unicorn. According to
this hydrogen can improve lean burn characterizes of methane engines, but only if it consists of at least 20% of the total mixture (I'm not sure if that's mass or volume.) With that being said I doubt gasoline would be much better. If you'd need 20% hydrogen and 80% gasoline to make the hydrogen effective as a lean burn catalyst you'd need one hell of a super efficient hydrogen generator in order to outweigh the losses and
at the same time the engine better be super efficient, like 50% efficient, to begin with. In which case you're not likely going to get any more efficiency anyway.
As I keep doing the math, if hydrogen can help create a lean burn gasoline engine it would likely be much more efficient and therefore cheaper to simply buy tanks of hydrogen and hook them up to your car. Propane can burn down to a 30:1 AFR (spark ignited) and it's more readily available, so I think I'll start with that.