I think it was posted once before but it's all in the maths.
How much power does it take to produce 1 ltr of H2, the "short" answer would be 286 kj. 
Meaning 1 ltr of pure hydrogen would take 333 minutes or 5.56 hours and require 5.56 watthours of energy.
To just equal the power output of just 1ltr of standard ULP it would take 297.97l of hydrogen.
If you were then to work that out to a 15% ratio over 1 Disco tank of 80ltrs your talking about 3576ltrs of hydrogen meaning that at that rate it would take 2.28 years to fill the H2 tank for 1 fill of the petrol tank, meaning by my laymans calculations would need 1.022736Mj of energy.
Some how I don't think the standard alternator will do the job, it may need an upgrade :D
It's nothing to do with personal feelings, opinions or attitudes why the home made, run off the engine systems referred too don't work, it is the physical science and maths which make it impractical.
If you want some link to verify my calculations, I found this on a Google search, came up first link when I punched in "How much power does it take to make 1 litre of Hydrogen gas".
How much electric power you need to produce 1 litre of hydrogen from water? - Yahoo! UK & Ireland Answers
My answer is not just based on that, it's also based on my Mechanical experience (22 years qualified motor mechanic) and my electronics background (just a couple of certs and a Diploma, nothing much).
If anyone smarter than me checks my numbers and I'm wrong please point it out as I'm not a mathematician or an engineer.
Cheers Casper.