The Water-Gas shift occurs at around 800deg C, whereby H2O(water) and CO (carbon monoxide) reform into H2(Hydrogen) and CO2. So theoretically, if you had a rich combustion environment, so you had a surplus of CO, you could inject water and get some additional reactions:
C8H18 + O2 ---> CO + CO2 + H20
CO + H2O ---> CO2 + H2
H2 + O2 ---> H2O
The tricky part would be the oxygen. Since in order to end up with free CO molecules to fuel the Water-Gas shift, you would have to be deficient in O, where do you get the O needed to combust your H2???
You would need to have a twin, one cylinder with water injection and running really rich, the other cylinder running on a mixture of the exhaust from the first cylinder and more fresh air. Then again we all know what happens when you mix hot, fuel laden exhaust gas with fresh air... no spark ignition required!