Another big thing to remember is that hydrogen doesn't explode. Ever.
Nope, hydrogen implodes, it's products of combustion much more dense than the fuel and oxidizer themselves. You'll get a bit of pressure from the heat released by combustion warming the products, but not nearly as much as you get from burning hydrocarbons, which pack more energy in a denser package.
It takes a lot of hydrogen to generate much pressure. Which is yet another reason why hydrogen/gasoline "flex fuel" vehicles get such horrifically poor economy on hydrogen. You thought the hit to economy on E85 was bad? Haven't seen anything yet. :roll:
Something that nobody's explained to me yet, though. Okay, so hydrogen's supposed to have an incredibly fast flame front (I'll give them that), allowing more rapid and complete combustion in a homogeneous fuel/air charge where your flame front speed dictates how rapidly you set fire to the rest of the fuel. But in a diesel you don't have a homogeneous charge, you have a stratified charge. At its fastest you can ignite the fuel as soon as it enters the chamber.
So what would be the benefit of having a homogeneous hydrogen/air charge with stratified injection? Your hydrogen's going to light off when the diesel first ignites, flame's going to spread nigh-instantaneously across the chamber, and you're still going to be injecting diesel when that hydrogen flame shuts out.
Sure, you raise the chamber temp a little bit, but it's not going to get hotter than the point temperatures right there at the injectors, where you have much more dense diesel gobbling up all the available oxygen to produce this gorgeous hotspot, so it won't help ignite the diesel injected during or after the hydrogen flamefest.
At best I can see it reducing NOx emissions by reducing the free oxygen in the charge before the heat really gets cranked up into NOx-forming ranges. But for those of you into the whole "greenhouse gasses" thing remember this: water vapour has been declared a more powerful "greenhouse gas" than CO2. Your H2O emissions are going to skyrocket if you're burning hydrogen in your engine.