MDI has been trying to get these vehicles into regular production for years. I think it can work, although I'd also have a bit of concern to have such a large very high pressure tank on board. The range-extending use of liquid fuel is relatively new thing for MDI, if I'm not mistaken. If I understand it right, they don't burn the fuel in the air motor pistons, so it shouldn't be too picky about the type of fuel used. That would mean that all kinds of alternative fuels (bio diesel, ethanol, methanol, butanol) could become usable in the same car! And LPG could also be used without too much trouble.
MDI's lightweight construction methods would also be very helpful to make a practical high-efficiency vehicle that has a small-displacement conventional engine. Turbo 500cc sport coupe? Turbo 1-Liter Minivan? Maybe!
Another kind of compressed air hybrid has been prototyped in the heavy truck world. It consists of a conventional diesel powertrain that is supplemented with a hydraulic pump/motor and pressure "accumulator" (compressed gas tank... air or pure nitrogen). During braking, the vehicle energy of motion is converted by the hydraulic motor into stored pressure in the accumulator tank. When the vehicle is ready to go, pressure is released and the hydraulic motor converts it into propulsive force to boost the off-the-line acceleration, reducing the demand on the diesel motor. It's a lot like the Prius or GM-two mode hybrid systems, except that you have hydraulic pump/motor in place of the electric generator/motor and a pressure tank in place of an electric battery pack. Sometimes this is referred as a "
hydraulic hybrid". And would be really good for frequent start-and-stop driving situations like city passenger busses and garbage trucks. In the systems prototyped so far, I think the pressure capacity is only good for a small number of starts or stops, so this is not a long-distance vehicle technology, but it would really help some of the lowest-efficiency working vehicles that there are!