I mentioned Jason Fenske's explanation in this thread:
http://ecomodder.com/forum/showthrea...cci-36404.html
After googling around I found that propane (and possibly natural gas) kits for diesel engines were a real thing, mostly used for more power (and inevitable damage) but could (barely) increase efficiency. I was mainly interested in natural gas as it is relatively abundant right now in the USA.
These systems don't appear to have near the control of control over the propane inserted into the airflow and don't appear to have much more control over the existing ECU (presumably the driver adjusts the air-fuel ratio with his right foot as you start with a diesel engine).
If you wanted to get the effects from the RCCI research, you would probably have to hack a megasquirt to handle two complete ignition systems (probably buy two and have a master/slave system where the diesel megasquirt gives the gasoline one specific fuel injection levels and otherwise ignores it).
If it works at all I'd expect to see it in commercial trucks. I'd expect them to deal with adding fuel from two pumps for a 20% increase in fuel efficiency, followed by truck stops having gas/e85 next to diesel pumps at truck stops.
My guess is that existing propane kits are unlikely to increase efficiency enough to bother without heavy megasquirt hacking. How much you are into that is up to you. One other interesting thing is that most of the research centered on a single RPM: this is close to commercial truck practice, but also of interest to solar enthusiasts who want generator backup. Presumably 1800 rpm would be ideal (you really need 3600, but you could design the generator with more poles or possibly gear it. The efficiency of 1800 should outweigh the gearing issue).