The problem with electric/hybrids is that it ends up adding alot of weight to a car or making it prohibitively expensive or unprofitable. The hybrid system doesn't do that much for highway driving (as you can see above), and all that weight adds to the city cycle stop and starting, given the efficencies of battery, regenerative braking and electric motors you end up with not a great increase. I wonder if Honda had just built a 3 cylinder 1 litre car with direct injection and tossed out the hybrid stuff if it would have done even better. It certianly would have been cheaper.
Direct injection has been used on two stroke engines, Diesels, Gasoline/petrol, etc. But I haven't as yet heard anything about any developments with gaseous fuels like propane or LPG etc. I don't know if that is because of technical problems with direct injection, or that it is not economically useful to do so. I would guess atleast some of the benifits would translate to LPG applications.
I would guess using off the shelf injector systems would have limitations, but I am guessing this could be modified or creatively worked around, certainly standard LPG systems seem pretty primative to me. Much room for improvement.