I assumed he meant simultaneous injection; I've also heard of adding propane, or for the more adventurous and those with better lined pockets, compressed natural gas, to a diesel engine for better performance.
You'd be interested in this link I bet:
H-Line Conversions - Hummer H1 & H2 Diesel and Green Fuel Conversions - Wichita, Kansas
He does a lot of bigger cars, but how hard is it to downsize a bit?