Do you understand how the electronic diesel injector works, it still is a pressure system, it just uses an electrical switch to open the injector. So how do you compress the gas and the diesel and control the mix.
You talk like this is some theory. Diesel engine manufacturer have offered dual fuel systems with Natural Gas for decades. I have participated in the engineering/construction/startup of these in remote locations for the past two decades for oil/gas industry.
One example of some bigger dual fuel engines:
http://www.wartsila.com/en/power-plants/power-generation/dual-fuel-power-plants
The mixture is controlled by controlling fuel rail pressure, and having calibrated fuel injectors for each cylinder. A realtime control system resolves the crankshaft position continuously. Fuel timings and quantities are controlled by when the injector is opened relative to each pistons position in its cycle, and how long it is held open. There are A LOT of other factors involved, but that's it in a nutshell
CNG and propane can work well in diesel engines. If the current boon in natural gas continues and the eco-weenies don't put a clamp on it, it'll be a good direction to go.