If the MAF is right before the propane injectors, pre-turbo, a boost leak won't matter for AFR metering - the MAF is accurately reading air moving past the injectors and the boost leak is actually leaking mixed air/fuel, so it wouldn't affect AFR. A boost reference is required so that
a) you can compensate fuel pressure for the air pressure where the injectors are (which isn't an issue pre-turbo) and
b) if you are injecting AFTER the turbo, where a leak would cause a misreading on the air passing the injectors, it can be corrected
Direct injection is way outta my field - I've always disliked the way a diesel engine works for whatever reason...spontaneous combusion just seems less controllable/precise/tunable. I just don't like it
lol
I think using actual injectors (be it liquid or vapour) mounted to the intake pipe right before the turbo would give me the best control via ECU. Although, injectors would have to be pulsed instead of just constantly flowing fuel into the stream of air like a carb.
Hmmm...I wonder if that would have a significant impact on how well the air and fuel mix together. Given the fact that its all going to get pushed through a turbo and 10 feet of piping, I think the difference would probably be negligable.